Geology

Geology (Gr. gē, 'the earth;' logos, 'a discourse') is the science of the earth—that science, namely, which has for its object the study of the various constituents of the earth's crust, with a view to discover how those materials have been aggregated and caused to assume the appearances which they now present. Geology, in short, is an inquiry into the history and development of the earth's crust, and of the several floras and faunas which have successively clothed and peopled its surface. As a science geology is comparatively young, although it can hardly be doubted that from a very early period the phenomena with which it deals must have claimed some attention. It is easy, indeed, to trace in old mythologies and legends the influence of the geological features of the land upon the human imagination. Volcanic eruptions, earthquakes, avalanches, and landslips, the havoc of torrential waters, and the destructive action of waves and breakers have unquestionably left their impress upon the superstitions and beliefs of all primitive peoples. One may believe that many of the remarkable scientific premonitions which are met with in oriental cosmogonies and the early writings of the Greeks may have been suggested by geological phenomena. The occurrence of sea-shells in the rocks of mountains and regions far removed from the sea may well have given rise to the oriental belief in the alternate destruction and renovation of the world. Pythagoras and Strabo both recognised that changes had taken place on the surface of the earth, but neither appears to have got beyond the observation of a few obvious phenomena—their explanations of which are hardly entitled to be considered more than vague guesses. It is not until we reach the close of the 15th century that we find geological phenomena attracting the attention of competent observers. With the investigations of the celebrated painter, Leonardo da Vinci, together with those of Fracastoro, a new departure was taken. The numerous fossil shells discovered in engineering operations were appealed to by them as evidence of former geographical changes—their method of reasoning being consistent and logical. Unfortunately it did not convince either their contemporaries or immediate successors—some of whom held the extraordinary view that shells and other fossil organic remains were not really what they appeared to be, but the result of a plastic force which had somehow fashioned them in the bowels of the earth. Fossils were further supposed to be the results of the fermentation of fatty matter, or of terrestrial exhalations, or of the influence of the heavenly bodies, or, finally, to be simply earthy concretions or sports of nature. Others, however, while maintaining that fossils were in truth the relics of formerly living creatures, held the opinion that all these had been buried at the time of the Noachian deluge. This controversy lasted for more than a hundred years, but long after the true character of fossils had become generally admitted their entombment in the strata continued to be attributed to the action of the deluge. This belief prevailed through the 17th and 18th centuries, and sadly interfered with the growth of geology; the prolonged infancy of which must be largely attributed to its influence. Steno, a Dane, who lived in Italy in the middle of the 17th century, would appear to have been the first to observe a succession in the strata. Hitherto stratified rocks had not been differentiated; they were all lumped together as representing the tumultuous deposits of the Noachian deluge. Steno, however, distinguished between marine and fresh-water formations, and showed that there were rocks older than the fossiliferous strata in which no organic remains occurred. Nevertheless, this clear-sighted observer could not free himself from the fashionable hypotheses of his day. While a belief in the universality of the Noachian deluge was prevalent, many strange 'theories of the earth,' such as that by Bishop Burnet, saw the light. These showed not only how the world had been evolved out of chaos, how it fared before, during, and after the deluge, but in what precise manner it was eventually to be wound up and consumed. The 'theories' referred to differed in detail, but their imaginative authors agreed in the notion of an interior abyss, whence at the time of the Noachian catastrophe the waters rushed, breaking up and bursting through the crust of the earth to cover its surface, and whither, after the deluge, they returned again.

Leibnitz (1680) proposed the bold theory that the earth was originally in a molten state, and that the primary rocks were formed by the cooling of the surface, which also produced the primeval ocean by condensing the surrounding vapours. The sedimentary strata resulted from the subsiding of the waters which had been put in motion by the collapse of the crust on the contracting nucleus. The process was several times repeated until at last equilibrium was established.

Hooke (1688) and Ray (1690) considered the essential condition of the globe to be one of change, and that the forces now in action would, if allowed sufficient time, produce changes as great as those of geological date. In Italy, Vallisnieri (1720), Lazzaro Moro (1740), and his illustrator, Cirillo Generelli, taught that there had been depressions of the land, during which marine fossiliferous strata were deposited, and that subsequently the seabottom had been elevated by the subterranean forces, and converted into dry land. Moro maintained the impossibility of the whole earth having been covered by the waters of the sea up to the tops of the highest mountains. The continents, he said, had been upheaved, and the fractures and dislocations of the strata were pointed to in confirmation of this view. Generelli insisted upon the gradual degradation of the land by running water, and held that the waste was so great that eventually the mountains must be washed down to the sea. This inevitable degradation of the surface, however, would be counterbalanced, he inferred, by elevation of the land elsewhere. But as Italian geologists, in common with those of other countries, believed that the world was only some 6000 years old, Moro and Generelli found some difficulty in explaining how so many thousands of feet of strata could have been accumulated within the limited period allowed by the orthodox chronology. They suggested, therefore, that the materials entering into the formation of the strata had been largely derived from volcanic eruptions.

Eventually the more advanced views held in Italy spread into France, Germany, and England. Buffon (1749), by the publication of his Theory of the Earth, evoked a spirit of inquiry in France; Lehmann (1756), Fuchsel (1762), and others in Germany did much to establish more correct methods of observation and interpretation of geological phenomena than had hitherto prevailed; while in England a distinct advance was made by Michell (1760) in his essay on the Cause and Phenomena of Earthquakes. The next name that comes into prominence is that of Werner, professor of Mineralogy at Freiburg in Saxony (1775). This celebrated writer framed a classification or system of the rocks of the Harz Mountains, in the order of their succession, and consequently in that of their formation, and maintained that this order would be found to prevail generally throughout the world. Werner's classification has proved inadequate, and even in many respects erroneous. Nevertheless, to him belongs the great merit of having brought into prominence a definite principle in the construction of the earth's crust, and a precise method of geological investigation. This discovery of the fact that strata occur in a certain order of superposition had been anticipated by several Italian geologists, and by Lehmann in Germany, but Werner's fame as a brilliant investigator and attractive teacher overshadowed and eclipsed the most of his predecessors. In some respects the views of this eminent man were retrograde. He maintained, for example, that his 'formations' were universal, and had been precipitated over the whole earth in succession, from a common menstruum or chaotic fluid. The igneous rocks, according to him, were chemical precipitates from water; he believed that no volcanoes existed in the earlier ages of the world, but that volcanic action was exclusively of modern date. Yet the true nature of igneous rocks had already been recognised in Italy, France, England, and Germany. With the publication of Werner's views on this subject a great controversy began, which was carried on with an acrimony that is now hard to realise. Those who upheld the igneous origin of such rocks as basalt were styled Vulcanists, while those who followed Werner became known as Neptunists. The great apostle of Vulcanism in Britain was James Hutton (1788). He not only insisted upon the igneous nature of basalt rocks but demonstrated in the field that granite likewise was of igneous origin. This philosophical thinker deprecated the calling-in of hypothetical causes to explain geological phenomena. The only agents of change, according to him, were those which are now at work in modifying the earth's crust. The past, therefore, was to be interpreted through the present. It was only through our knowledge of the methods employed by nature in carrying on her operations in our own day that we could hope to interpret the record of the rocks. The Huttonian theory was fortunate in having for its expounder John Playfair, whose famous Illustrations (1802) has long been held in the highest esteem, and is still studied by geologists. Another friend and disciple of Hutton, Sir J. Hall, became the founder of experimental geology, and did much towards the establishment of the cardinal doctrines of his teacher. Hutton's observations were confined to Scotland, in which fossiliferous strata are not prominently developed. It was the igneous masses—the crumpled and shattered rocks of mountain and glen and sea-coast, and the never-absent evidence of denudation and decay that fascinated him. He saw 'the ruins of an older world in the present structure of the globe,' but he knew nothing of that long succession of ruined worlds, each characterised by its own life-forms, with which William Smith (1790) was shortly to astonish geologists. This able investigator alone and unaided had explored all England on foot, and succeeded in completing a geological map of the country on which the strata were for the first time delineated and thrown into natural divisions. His views as to the law of superposition among strata were arrived at independently of Werner, and he was the first to point out how each rock-group was distinguished by its own peculiar fossils. Hence Smith is justly entitled to be called the founder of historical or stratigraphical geology. Since then the progress of geology has been rapid. Fossils which at first were valued chiefly as marks by which one formation could be distinguished from another by-and-by claimed fuller attention—the classic researches of Cuvier in the Paris basin forming a great epoch in Palaeontology (q.v.), or the study of fossil organic remains.

In closing these remarks on the history of the geological sciences, it would be unjust to omit the name of Lyell, whose great Principles of Geology (1830-33) did invaluable service. His labours were based on those of Hutton and Playfair, but he carried out their doctrines further in some directions than either of these geologists were prepared to go, while in other directions he did not advance so far. Before the appearance of Lyell's well-known work, the Huttonian philosophy had conspicuously triumphed, but geologists were still prone to account for what appeared to be 'breaks in the succession' by the hypothesis of vast catastrophes. They conceived the possibility of world-wide destruction of floras and faunas, and the sudden introduction or creation of new forms of life, after the forces of nature had sunk into repose. The full meaning of denudation had not as yet been generally appreciated, and subterranean action was still frequently appealed to in explanation of orographic features which are now recognised to be the work of epigene action. Such views gained for their upholders the name of Cataclysmists or Catastrophists. Lyell's main idea that the present is the type of all preceding ages, so far as these are revealed by the fossiliferous strata, has gained for his school the title of Uniformitarian. But within recent years many of his disciples have departed somewhat from the teaching of their master, and maintain that the operations of nature have been the same in kind, but not necessarily in degree. The impulse given to the advance of biological science by the publication of the Origin of Species (1859) has also affected geology, and not on its paleontological side alone. In the departments of physical and stratigraphical geology one may note a larger and broader method of treatment since the appearance of Darwin's famous work—the dominant tone in geological literature at present being rather evolutionary than uniformitarian in the narrow sense. Another distinguishing feature of geological science in our day is the great attention paid to Petrography (q.v.), the study of which had fallen into comparative neglect in this country for many years. Interest in it, however, was revived by Dr Sorby, who showed how much might be learned by examining thin slices of rocks and minerals under the microscope. The introduction of the microscope into petrographical investigation has thus opened up a wide and novel field of inquiry, from the assiduous cultivation of which much may be expected.

It may be interesting to point out as shortly as possible the order of development of the geological sciences. Unquestionably the earliest to take shape was Mineralogy—a work on descriptive mineralogy by Agricola having appeared in 1546. In fact, several complete treatises had been published before the middle of the 18th century. Geognosy, or the study of the various rocks of which the earth's crust is composed without special reference to the mode of their arrangement, was the kind of geology which chiefly occupied the attention of the earliest investigators. The term is now practically disused, and in its place we have Petrography. When employed by modern writers it has usually a wider signification (see GEOGNOSY). Structural Geology, or the mode in which rocks are built up in the earth's crust, next began to come into prominence, and Dynamical Geology, or the study of causes now in action soon followed—the system advocated by Hutton and Playfair being that which has gained general acceptance. Thereafter followed Experimental Geology, of which Hall was the father. Although some progress had been made by Lehmann, Fuchsel, and Werner in the method of determining the succession of strata and of grouping these in chronological order, yet Historical or Stratigraphical Geology can hardly be said to have existed as a science before the date of William Smith's classical researches. Paleontology is of still more recent origin, the names of Cuvier, Lamarck, and Brongnart being conspicuous among its earliest exponents.

A brief outline may now be given of the various departments of geology, properly so called.

DYNAMICAL GEOLOGY.—The modern system of geology is based on the principle that the past is to be interpreted through the present. In other words, the geologist believes in the constancy of nature, and that by studying the effects produced by the action of her various agents in the present he will be able to interpret the records of such action in the past. The study of such natural operations constitutes dynamical geology.

The various forms of energy from which geo- logical changes arise may be divided into two series—viz. hypogene action and epigene action.

Hypogene Action.—Under this head come the changes which are induced by the internal heat of the earth, those changes, namely, that are in progress beneath the earth's surface. In this category are included volcanoes and volcanic action, volcanic products, and the chemical and mechanical changes which are superinduced in such products and upon the rock-masses with which these come into contact during volcanic eruptions (see VOLCANOES). Lava (q.v.) and Tuff (q.v.) are studied as regards their composition, texture, and structure, while the manner in which these and other volcanic products are built up is likewise investigated. All this is done with a view to comparing such volcanic products with similar crystalline and fragmental rocks which occur in regions where volcanic action may have become quite extinct. Another most important set of hypogene phenomena are movements of the earth's crust. See EARTHQUAKES, UPHEAVAL, BEACHES (RAISED), SUBMARINE FORESTS.

Epigene action has reference to those operations that affect mainly the superficial portion of the earth's surface. The epigene agents are the atmosphere, rain, brooks and rivers, ice, the sea, and life. The effects of atmospheric action are seen in the general disintegration of rocks, the formation of Soil (q.v.), and the accumulation of dust and sand (see DRIFT). In the diffusion of life over the globe, wind has also no doubt played in all ages an important part. Rain, again, charged with the carbonic acid, &c., which it absorbs from the atmosphere and vegetable soil, acts chemically upon rocks—all of which are more or less permeable. Much rock-disintegration is thus induced, the 'weathered' materials being dispersed or accumulated locally by the mechanical action of the rain. The chemical action of rain is not confined to the surface of the ground, for much water filters down through natural cracks, fissures, &c., and is thus enabled to soak into the rocks at all depths. The underground water which is not absorbed in the interstitial pores of rocks rises eventually, and is discharged at the surface as Springs (q.v.), which are more or less impregnated with dissolved mineral matter abstracted from below. These springs are either cold or thermal, and constant or intermittent. In some volcanic regions the water comes to the surface in eruptive fountains (see GEYSER). The destructive action of such underground waters is seen in the excavation of caves, tunnels, and other subterranean passages (see CAVE), and in the production of Landslips (q.v.) and rock-falls; while their reproductive action is familiarly illustrated by the formation of Stalactites and Stalagmites (q.v.), and the accumulation of great masses and sheets of siliceous Sinter and Calcareous Tufa (q.v.). Brooks and rivers act as potent agents of change. By means of the detritus which they sweep along or carry in suspension, they rub, grind, and erode the rocks over which they flow, and thus in time ravines and valleys have been excavated. The eroded materials are constantly travelling from higher to lower levels until they come to rest in lakes or the sea. Hence lakes and the sea in many places are being gradually silted up—the growth of Deltas (q.v.) being one of the most notable evidences of epigene action. The action of rain and running water is greatly aided by frost, which is a powerful disintegrator of rocks. Water freezes as well in the minute pores of rocks as in the fissures by which rocks are traversed, and thus when thaw ensues the loosened grains and particles are ready to be carried away by wind, rain, and melting snow; while disjointed blocks, &c. may fall asunder and topple from cliffs or roll down steep slopes. In regions of perennial snow-fields the avalanche and the glacier likewise act as important denuders of the surface, and transporters of rock-debris from higher to lower levels (see AVALANCHES, GLACIERS, BOULDER-CLAY, &c.). Again, in certain latitudes lake and river ice are conspicuous agents of change—acting especially as rafts for the transport of stones and debris (see ANCHOR-ICE). Thus the whole surface of the land from the highest mountains down to the sea is being gradually degraded or lowered by the combined action of many epigene agents. There is a continual and universal disintegration of rocks going on, and a no less continual transport of material and building up of this into new formations. Alluvial flats and terraces, deltas, &c. may be cited as prominent examples of the sedimentary series of modern accumulations, while the chemical series is well represented by the calcareous formations of springs and brooks, and the precipitations of common salt, sulphate of lime, &c., which are taking place in saline lakes (see LAKES).

The sea as a geological agent acts in three ways: it erodes rocks, and transports and accumulates sediment. The work of erosion is confined for the most part to that marginal belt within which waves and breakers work. These by means of the shore-detritus batter and undermine cliffs, and cause them gradually to recede, and hence the sea may be said to act like a great horizontal saw. The materials brought down by rivers or detached from the shore by the action of the sea itself are distributed by currents over the sea-floor, the coarser detritus gathering in shallow water, while the finer sediment is swept out to greater depths and spread over wider areas. Such terrigenous materials extend outwards from the shore to a distance of 60 to 300 miles, and to depths of 2000 feet or more. They are confined, therefore, to a comparatively narrow belt of the sea-bottom. Over the abyssal depths of the sea, the only accumulations in progress are organic ooze and a peculiar red clay which is believed to be the result of the chemical action of sea-water on products of volcanic origin (see ABYSMAL ACCUMULATIONS). Now and again, stones and debris may be carried out to sea by icebergs and dropped beyond the zone of terrigenous sedimentation. Similarly, rock-fragments entangled in the roots of trees or buoyed up by seaweeds may now and again come to rest in abyssal regions. Reference has been made to the geological action of the ice of lakes and rivers, but the icebergs and ice-rafts of high latitudes must not be omitted. Much rock-debris is distributed by such agencies over the sea-bottom, detached fragments of the 'ice-foot' (see ICE, vol. vi., p. 59) being the most notable carriers of stones.

The action of plants and animals is not ignored by geologists. Plants aid in the disintegration and rupture of rocks by means of their roots and the organic acids derived from them during decay. Rocks are drilled and bored by some kinds of marine molluscs, annelids, echini, and sponges, and are thus weakened and more readily yield to the action of waves and breakers. Burrowing animals also bring about changes, the common earthworm being an efficacious agent in the formation of soil (see EARTHWORM). Plants occasionally act as conservative agents, as in the fixing of blown sands (see DUNES), and in protecting the banks of lakes and rivers. Again, forests, by equalising and regulating the flow of the water of precipitation, prevent the destruction of soils and subsoils by torrential action. In some regions also the rocks along a seashore are partially protected from the waves by seaweed, sponges, zoophytes, and gregarious molluscs. Amongst formations of organic origin may be mentioned soil (in part), peat-bogs, morasses, mangrove-swamps, bog-iron ore, &c.

Some calcareous algæ also form considerable beds, as among the reefs of the Florida seas; while certain marsh-loving and fresh-water plants have the power of abstracting carbonate of lime from water and encrusting themselves therewith. Thick masses of calc-tufa have originated in this way. The organic oozes of the deep seas are good examples of deposits formed of the exuviae of minute pelagic organisms; and the great coral-reefs (see CORAL) of the warmer oceans are still further evidence of the importance of life in the production of new formations. Such are some of the accumulations which are almost wholly composed of organic debris; but animals and plants contribute to the growth of many other deposits. The marine terrigenous formations are charged more or less abundantly with the relics of animal and plant life; nor are similar remains wanting in the alluvial deposits of rivers and lakes.

PETROLOGY.—From the study of causes now in action the geologist learns that many of the rocks, with which every one, whether observant or not, necessarily makes some acquaintance, are of the same character as epigene and hypogene products. For a particular account of the rocks themselves, PETROGRAPHY and the articles therein cited may be consulted; here all that can be attempted is to point out very briefly how far a knowledge of formations now in progress enables us to explain the nature and origin of rocks.

(1) Igneous Rocks.—In Great Britain and other countries where at present there is no volcanic action we meet with various glassy rocks, such as pitch-stone and obsidian, with semi-crystalline rocks, as trachyte, phonolite, liparite, andesite, basalt, &c., with crystalline rocks, such as certain dolerites, and with fragmental rocks, like tuff and agglomerate, which in every essential particular resemble the products of modern volcanoes. But, as might have been expected, the older igneous rocks are often more or less altered, such alteration having been superinduced by the chemical action of percolating waters, by pressure, by crushing, or by these and other causes combined. There is a class of crystalline rocks, however, which, although they consist of the same mineral ingredients as occur in many igneous rocks, yet differ so materially in character from lavas that geologists are warranted in believing that they could not have been consolidated at or near the surface of the earth. This class is represented by such rocks as granite, syenite, gabbro, and certain diorites, dolerites, quartz-porphyries, &c. A study of these rocks under the microscope and in the field as rock-masses leads to the belief that they are indeed of igneous origin, but have cooled and consolidated at some depth in the earth's crust, their appearance at the surface being due to subsequent denudation. Thus two classes of igneous rocks are recognised—viz. volcanic or superficial, and plutonic or deep-seated.

(2) Derivative Rocks.—Under this head are included all the products of epigene action. They are termed derivative inasmuch as most of them are composed of materials which have been derived from pre-existing rocks by the chemical or mechanical action of epigene agents, while others are made up of organic debris. They may be roughly classified as follows:

Mechanically-formed Rocks.—These consist of fragmental materials. They are granular non-crystalline aggregates, the constituent ingredients of which may be angular or rounded in form, and may or may not be arranged in layers. They consist of (a) Eolian or Aerial rocks, such as blown sand (dunes) and dust-deposits. The products of the 'weathering' action of the atmosphere, such as rock-debris (breccia), certain clays, &c., are also in part of eolian origin. (b) Sedimentary rocks, as conglomerate, breccia (in part), sandstone, graywacke, various clays, mudstones, shales, &c. (c) Glacial rocks, as rock-debris, erratics, moraines, boulder-clay, &c.

Chemically-formed Rocks.—The rocks included under this subdivision are sometimes earthy in character, but more frequently show a crystalline or compact sub-crystalline texture. Among the more typical kinds are kaolin and various other clays, stalactites and stalagmites, calc-tufa and its varieties, geyserite (siliceous sinter), rock-salt, dolomite, gypsum, flint, chert, various ironstones, &c.

Organically-derived rocks are made up of the relics of animal and plant life. They include a great variety of limestones, diatom-earth (tripoli), flint (in part), various phosphatic deposits, peat, lignite, coal, anthracite, oil-shale, various iron ores, &c.

No hard and fast line can be drawn between the older and younger products of epigene action. It is obvious that conglomerate and sandstone are merely compacted gravel and sand; breccia is only consolidated rock-debris; while lignite and coal are simply vegetable matter more or less mineralised. The thick fossiliferous limestones of the earth's crust are paralleled by the coral-reefs and organic oozes of existing oceans, and have evidently had a similar origin. Every derivative rock, indeed, can be compared with a like product of modern epigene action. The older products, it is true, are most frequently solidified, while the younger are oftener more or less incoherent and unconsolidated. But this difference is not essential, and is only what might have been expected. The older products have for a long time been exposed to the action of percolating water. In many cases they have been subjected to the influence of subterranean heat and enormous pressure, and we need not wonder, therefore, that they should have acquired a more or less indurated character. But solidification does not invariably characterise the older products, nor are modern accumulations always incoherent. There are indurated conglomerates and sandstones of very recent formation, and some modern coral-rock is as hard and compact as the older limestones. Hence the term rock is applied to all the products of epigene and hypogene action alike, whether the material so designated be yielding, as clay and peat and blowing sand, or hard and resisting, as conglomerate, limestone, or granite.

(3) Metamorphic Rocks.—All rocks sooner or later undergo some process of alteration whereby their original character becomes modified. Thus, by the chemical action of percolating water some limestones have been more or less changed into dolomite; olivine rocks have been altered into serpentine; some sandstones have been converted into quartzites. Derivative rocks at the point of contact with igneous rocks are very frequently altered to a greater or less extent. Thus, ordinary limestone becomes crystalline marble, coal is changed into graphite, sandstone into quartzite, clay and shale into porcelainite. When alteration of a rock, however caused, has proceeded so far as to produce a rearrangement of the constituent elements of a rock, and to develop a crystalline or semicrystalline structure, such extreme alteration is termed metamorphism, and the rocks so affected are described as metamorphic. Rocks of this kind are sometimes confusedly crystalline or massive in structure, and in hand specimens might be mistaken for plutonic igneous rocks; but by far the larger number are distinguished by a peculiar flaky or pseudo-laminated structure which is termed Foliation (q.v.). In foliated or schistose rocks the constituent minerals are arranged in alternate lenticular layers which merge into each other. Such arrangement, it must be understood, has no relation to the layers of deposition so frequently present in derivative rocks like shale, sandstone, &c. The foliated structure has been superinduced in rocks, some of which may have been igneous and others aqueous in origin. It is obvious, however, that the study of causes now in action can throw little light on the origin of foliation. We may study the changes induced in rocks by contact with the products of modern volcanic action, and these will doubtless enable us to understand how certain alterations in rocks have been brought about; but schistosity is not superinduced in rocks in the neighbourhood of modern volcanic orifices. In Britain and other countries, however, denudation has exposed the interior and basal portions of ancient volcanoes, and we can now study in detail the fractured and baked rocks through which heated gases, molten matter, &c. have been erupted. Nay, in some cases, we can even examine enormous masses of plutonic crystalline rock which are believed to be the reservoirs from which the molten matter of our ancient volcanoes was pumped to the surface. Such great plutonic masses are frequently surrounded by a zone or belt of crystalline schistose rocks, such as gneiss, mica-schist, &c. The rocks are most crystalline and schistose in the immediate proximity of the igneous mass, but gradually lose these characters as they recede from its neighbourhood, until by-and-by they pass into ordinary derivative rocks such as graywacke, shale, &c. Some schistose rocks, therefore, undoubtedly owe their origin to contact with deep-seated igneous masses. Again, it has been observed that where rocks, whether igneous or derivative, have been subjected to enormous crushing and pressure, they not infrequently become crystalline and schistose. There are some schistose rocks, however, the origin of which is still very obscure. Geologists cannot yet assert, therefore, that all schistose rocks are metamorphic (see ARCHÆAN SYSTEM). Among the most characteristic metamorphic rocks are quartzite, marble, phyllite, mica-schist, talc-schist, chlorite-schist, hornblende-schist, actinolite-schist, gneiss, granulite, eclogite, &c.

STRUCTURAL or GEOTECTONIC GEOLOGY is that branch of the science that deals with the arrangement or structure of rock-masses.

Structure of Igneous Rocks.—Igneous rocks are grouped under two series—viz. (a) Contemporaneous and (b) Intrusive eruptive rocks. (a) Contemporaneous eruptive rocks are either crystalline or fragmental. The crystalline rocks are simply old lava-flows, while the fragmental rocks consist of tuff and its varieties. They are in short the products of volcanic action, and have been erupted at the earth's surface, accumulating either upon the land or under water. Many of these rocks have apparently been erupted from vents of the ordinary modern type, but others appear to have come up along lines of fissure in the earth's crust—the lavas overflowing the surface in broad floods. Successive outflows of this kind, accompanied frequently by the ejection of fragmental materials, have built up some great plateaus. Contemporaneous lavas are generally more or less scoriaceous or porous above and below. (b) Intrusive eruptive rocks are also crystalline and fragmental. Necks are approximately cylindrical funnels filled with either crystalline igneous rock or fragmental materials, or with both. They are obviously the plugged throats of old volcanoes, the upper parts of which have been removed by denudation. Intrusive Sheets are more or less lenticular masses of crystalline igneous rock which have been erupted amongst strata in a direction more or less closely conformable with the planes of bedding. They seldom show any scoriaceous structure, and generally bake and alter overlying as well as underlying rocks—thus clearly indicating their subsequent origin. Dykes (q.v.) consist generally of crystalline rock which has been erupted in approximately vertical and even-sided fissures, thus giving rise to wall-like intrusions. Occasionally fragmental igneous rocks, such as agglomerate, are met with in similar positions. Veins is the term applied to smaller irregular and more or less tortuous intrusions of crystalline rock. Bosses (see NECK) are amorphous masses of crystalline rock, rising more or less vertically through surrounding rock-masses. There is reason to believe that many of these 'bosses' are the deep-seated reservoirs from which volcanoes were supplied with lava. 'Dykes,' 'veins,' and sometimes 'sheets' proceed from them into the adjacent rocks, which are often much altered and metamorphosed.

Structure of Derivative Rocks.—The most characteristic feature of these rocks is their bedding or stratification—a structure which is due to the mode of their accumulation. Hence they are often spoken of as the 'stratified rocks.' But, as we have seen, stratification likewise characterises contemporaneous eruptive rocks. As far the larger number of derivative rocks are simply aqueous mechanical and chemical sediments, they are also often termed 'aqueous' and 'sedimentary rocks.' Individual beds in a group of strata are lenticular or wedge-shaped; so that when any particular stratum is followed in one direction it eventually thins away and dies out. And the same is the case with groups of strata. Fine-grained deposits such as shale and limestone tend to be more persistent and to cover wider areas than sandstones and conglomerates. Almost any diversity of strata may occur in a group or series, but it is more usual to find certain kinds of rock associated together; thus, fine sandstone alternates with shale, conglomerate with grit, limestone with fine shales, &c. Again, individual beds are often found to change their character as they are followed in certain directions. Conglomerate, for example, passes laterally into sandstone, sandstone becomes argillaceous and passes into shale, while shale, by the gradual increase of calcareous matter, becomes marly and often passes into limestone. Sometimes the stratification is extremely regular, at other times the beds thicken and thin out very irregularly, and not infrequently they show what is called false-bedding or current-bedding—a structure which is seen both in aqueous and eolian accumulations (see DUNES). Amongst the surface-markings seen in sedimentary rocks the most common are ripple-marks, sun-cracks, rain-prints, and tracks, trails, burrows, &c. of worms, crustaceans, molluscs, reptiles, birds, &c.

Strata are not often quite horizontal; they usually dip at a less or greater angle, and such inclined strata are as a rule the remaining portions of large curves or undulations, the upper portions of which have been removed by denudation, so that the truncated strata crop out at the surface (see OUTCROP, STRIKE). The simplest form of curve assumed by strata is a monocline but anticlinal and synclinal folds occur much more frequently (see ANTICLINE). In strata with a moderate dip the strata on opposite sides of an anticlinal axis incline at approximately the same angle. But in more steeply inclined beds the dip is often greater on one side than the other, the beds on the steeper side of the fold becoming doubled in below their equivalents on the other side. This is what is termed 'Inversion'—a structure which when repeated gives us what are called 'Isoclinal Folds' (see MOUNTAINS). In regions of highly folded strata the fossils and even the stones in conglomerates are often flattened and squeezed out of shape. Such deformation likewise characterises whole rock-masses, as is well seen in the structure termed Slaty Cleavage (q.v.). As an extreme result of enormous pressure we occasionally find that clastic rocks have been converted into crystalline schists.

Most rocks, as well igneous as derivative, become gradually more and more consolidated. Soft incoherent sands and clays are compressed; lavas cool and harden. All rocks therefore tend to contract, and in doing so they become cracked, regularly or irregularly as the case may be. During the process of folding they have likewise yielded to stress and strain by cracking across. Such cracks are termed Joints (q.v.). But rocks are not only jointed; frequently they are traversed by great fissures of displacement called Faults or Dislocations (q.v.), which may sometimes be traced across the whole breadth of a country. That the phenomena of folding, fracturing, and displacement are the result of earth-movements cannot be doubted, and there is abundant evidence to show that such disturbances have taken place again and again, sometimes over limited regions, at other times over very much wider areas. This is proved by the phenomena of Unconformity (q.v.), in which one set of beds rests on the upturned and denuded ends of an older series.

The fissures and cavities of rocks are in some places filled up again by the introduction of various kinds of mineral matter through the chemical action of percolating water. In many cases such mineral deposition may have taken place from heated solutions, under great pressure, and at great depths from the surface. This is probably the origin of many of the Ore-deposits (q.v.) met with as lodes or veins.

PALÆONTOLOGICAL GEOLOGY.—A study of the physical characters of rocks enables the geologist to arrive at many interesting conclusions as to the mode in which rocks have originated. By such evidence alone it is sometimes possible to discover the successive changes which some particular region has undergone. Thus, the phenomena of igneous and glacial accumulations tell their own story, and even in the case of many sedimentary deposits geologists are able, without the aid of fossils, to distinguish between deep-sea and shallow-water strata; while certain rock-structures, such as unconformity, yield him evidence of changing physical conditions. Without fossils, however, investigations into the successive phases through which the earth's surface has passed could not proceed far: historical geology would be impossible. It is chiefly by means of Fossils (q.v.) that the deep-sea or shallow-water origin and the marine or freshwater character of strata are determined, and the climatic conditions under which they were deposited are ascertained. When we learn that many fossils belong to extinct species and even genera, and that different groups of fossils occur in different series of strata, it might seem, at first, as if this would tend rather to confuse than aid the geologist. But the cause of such apparent discrepancies lies, of course, in the simple fact that the fossiliferous strata belong to different ages—some are much older than others. In the uppermost or youngest series the organic remains approach most nearly to the life-forms of the present day, while in the lower and therefore older strata the fossils recede farther and farther from existing types as we follow them to lower and lower geological horizons. From this it would appear that there has been a gradual coming-in and dying-out of species, and observation has shown that when a particular flora or fauna has died out it never reappears in younger strata. When William Smith discovered that each well-marked group of strata was charac- terised by its own suite of fossils he had got the key to the history of a long succession of geological changes; for the fossils enabled him to recognise each group in whatever part of the country it occurred, and however much its petrographical character might have changed. If three conformable series of strata occur in the order A, B, C—B superimposed on A, and C upon B, that order is never reversed elsewhere. Each term of the series may not always be present—either one or more may be absent—but those that do occur always occupy the same relative position. In such a conformable sequence each group may contain fossils peculiar to itself, but a larger or smaller number will usually be found to range from one group to another, or even from top to bottom of the whole. The fossils will, in short, indicate a gradual change of fauna and flora, as we pass from below upwards—old forms disappearing, new forms appearing. But should the middle term of the series (group B) be wanting, then the passage from A to C, owing to the absence of the connecting forms belonging to B, will be more or less abrupt. A conformable sequence, like A, B, C, points to the persistence of similar physical conditions during a longer or shorter period. If the fossils in each group indicate a sea of moderate depth while the stratum attains a thickness of several thousand feet, the inference will be that sedimentation has taken place during a slow movement of subsidence. In other words, the silting-up of the sea has been retarded by the gradual sinking-down of its bottom. On the supposition that the accumulation of the strata has been a very protracted process, the marine fauna will have undergone more or less modification. Such change in the life-forms, however, will probably have been very gradual; some species remaining longer unmodified than others, while a few may persist unchanged through the whole period of sedimentation. In the case of an unconformable sequence—where C rests directly on A, the physical conditions have evidently not remained constant. After the deposition of A, a movement of upheaval has ensued; the sea has disappeared and land has taken its place. Should land-conditions have continued for a very prolonged period before subsidence supervened and the area once more became submerged, the marine fauna will, in the meantime, have undergone more or less modification in those regions to which it migrated while elevation was in progress. Thus the sediment (group C) which subsequently accumulated over the drowned land-surface would come to contain a suite of organic remains that might differ greatly from those occurring in the immediately subjacent group A. And the longer the interval between A and C, the more strongly marked would be the break in the succession of life-forms. Such 'breaks in the succession' are of common occurrence—local and more widely-spread movements of depression and elevation having characterised the formation of the fossiliferous strata everywhere. When it is remembered that every bed of aqueous rock has been formed out of the ruins of pre-existing rocks, igneous or derivative, or both, it is obvious that the fossiliferous strata cannot possibly contain a perfect record of all the forms of life which may originally have been entombed in sediment. Many fossils must have disappeared along with the rocks which contained them. Thus, in the case of such a 'break in succession' as that just described, it is obvious that the strata of group A would be more or less denuded before group C began to be accumulated—C would rest unconformably upon A. Nor can we believe that the life-forms of earlier ages were ever more fully represented by fossils than existing faunas and floras will be by the remains of living things which are now being buried in sediment. Of the myriads of existing terrestrial plants and animals how few will leave any relic behind them! Aquatic, and more especially marine forms, will doubtless be preserved in far greater variety and abundance; but amongst these are many delicately-fashioned and soft-bodied creatures which can only become fossils by accident, as it were. Such considerations as these should lead us to expect that the fossiliferous strata, even when these have apparently been accumulated in a continuous manner, will contain a most imperfect record of the past life-history of the globe. But notwithstanding this imperfection of the geological record there is yet ample evidence to show that gradual extinction of old and evolution of new faunas and floras has been the rule. Life has been persistent from its introduction, but subject to endless modifications. With this continuity in geological history it is obvious that any subdivisions of past time that we choose to make must be arbitrary, for the germ, as it were, of one so-called period must have begun in the period that preceded. But, just as in human history it is convenient to use such terms as the 'Middle Ages,' the 'Elizabethan Period,' &c., so in geology it is useful and indeed necessary, for purposes of description and correlation, to group the records into so many subordinate divisions. 'Unconformities,' 'breaks in succession,' &c. often enable this to be done with more or less ease; but in the case of the better-preserved portions of the stony record it is often very hard to say where a division-line should be drawn.

HISTORICAL GEOLOGY.—The forms of life that existed during some prolonged period of the past have a certain facies which serves to distinguish them as a group from the living things that flourished in preceding and succeeding ages. And the strata which contain such a well-marked assemblage of fossils are included under the term System. By this term, then, is understood all the deposits, whether terrestrial, fresh-water, or marine, which accumulated over the earth's surface upon land, in lakes, or in the sea, at a time when the world was characterised by the presence of some particular and peculiar fauna and flora. By comparing and correlating the fossiliferous strata throughout the world geologists have been able to arrange the various systems in chronological order. The following table shows the larger divisions and subdivisions in the order in which they would appear if they all occurred in one and the same section. (Each system will be found described under its own title.)

PHYSIOGRAPHICAL GEOLOGY.—Under this head is discussed the origin of the surface-features of the land—mountains, valleys, &c. The study of causes now in action shows that everywhere rocks are undergoing disintegration, the resulting detritus gradually travelling from higher to lower levels until eventually it reaches the sea. This continuous and universal denudation is easily read in the present appearance of the rocks forming the surface of the land. The phenomena of truncated strata, faults, &c. (see DENUDATION) demonstrate that thousands of feet of rock have been gradually removed in the form of detritus. To appreciate this fact some knowledge of structural geology is necessary. In regions which have long been exposed to denudation we recognise a very remarkable connection between the configuration of the ground and the nature and mode of arrangement of the rocks. The valleys and low grounds, for example, coincide in a general way with the distribution of the less durable rocks, while escarpments, hills, and ridges mark out the sites of the more resisting rock-masses. Again, in the case of undulating and folded strata, it most frequently happens that anticlines instead of forming hills give rise to valleys, while synclines correspond as a rule not to valleys but to hills. The reasons are obvious, for relatively hard rocks resist denudation better than softer rocks; and, while an anticlinal arrangement and the jointing of strata favour the action of the denuding agents, in the case of synclinal strata the rock-structure has just the opposite effect (see LANDSLIPS, MOUNTAINS). Thus the features impressed upon the land by denudation depend partly upon the composition and texture of the rocks, and partly upon their structure as rock-masses. In the case of a true mountain-range of recent elevation the larger features of the surface correspond in a general way with the folds of the strata. Thus the mountain-ridges often run in the direction of great anticlinal axes, while the long parallel valleys coincide with synclinal axes (see ALPS). But even in the case of mountains of elevation denudation has often profoundly modified such features. Anticlinal mountains are very unstable; rock-falls and landslips from time to time take place; and the tendency is for all mountains of that character to become effaced. Sooner or later the orographical features change, and are eventually determined by the epigene agents, directed and controlled by the composition and structure of the various rock-masses. Geologists recognise three kinds of mountains: (1) Mountains of Accumulation, such as volcanoes; (2) Mountains of Upheaval, such as true mountain-ranges like the Alps; and (3) Mountains of Circumdenudation, which owe their origin to the removal of material that formerly surrounded them, such as the mountains of the British Islands.

A plateau or tableland is simply an elevated plain, and may consist either of approximately horizontal sheets of rock, like the plateau of the Colorado, or of more or less highly folded and even contorted strata, which have been planed down to one general level, like the plateaus of Scandinavia and the Scottish Highlands. Both kinds of tableland are usually traversed by valleys, which have been excavated by running water, and sometimes, as in the case of the Scottish Highlands, they are so highly denuded that their plateau-character becomes obscure. Plateaus owe their elevation to upheaval, those which are built up of horizontal strata being termed plateaus of accumulation, while those which consist of folded and contorted strata are known as plateaus of denudation. Plains are only less elevated plateaus. Some of these, as, for example, the wide alluvial plains and deltas of great rivers, owe their origin to accumulation. Others, again, consist of low-lying land, the level of which has been reduced during a protracted period of denudation. Should such an area eventually be elevated it would become a plateau of denudation.

SPECULATIVE GEOLOGY.—There are certain great physical problems the data for solving which are more or less incomplete, or in the very nature of things beyond our knowledge. Amongst such is the question of the age of the sun's heat. This, of course, is rather a physical than a geological question, and yet geology furnishes evidence on the subject which the physicist cannot ignore. Some physicists are of opinion that the sun's heat is due to gravitation—that, as Sir W. Thomson remarks, the sun's matter, before it came together and became hot, may have existed in the condition of two cool solid bodies which collided with the velocity due to their mutual gravitation. If gravitation, therefore, be the only source of the sun's heat, that luminary cannot have been giving out heat at the present rate of radiation for a longer period than 20,000,000 years, or, as Professor Tait maintains, 10,000,000 years. But no geologist will admit that all the changes that have taken place on the earth's surface since the first appearance of life can possibly be included within such narrow limits. According to Dr Croll, however, the sun probably originated from the collision of two bodies moving directly towards each other with velocities greater than the velocities due to their mutual gravitation. As the heat generated by the impact of two such bodies would depend upon the velocity possessed by each before collision took place, it is obvious that the energy stored up in our sun may be infinitely greater than that which could have been derived from gravitation alone. So far, therefore, as a possible source of the sun's energy is concerned, Dr Croll is of opinion that life might quite well have begun 100,000,000 years ago.

Condition of the Earth's Interior.—This is another physical problem in the solution of which geology is necessarily interested. Several views have been advanced by physicists, the more generally received opinion being that the earth is a more or less solid globe. Others favour the hypothesis of a thin crust enclosing a liquid or viscous interior; while yet others think that a liquid substratum separates the crust from a solid nucleus. The appearance of volcanoes and thermal springs shows us that a high temperature exists beneath the crust, and similar evidence of internal heat is furnished by borings and mines. The mean of many observations shows that temperature increases 1° F. for every 54 feet of descent, so that if the temperature at the surface be 50°, the boiling-point of water (212°) will be reached at the depth of about a mile and a half. It is evident, therefore, that at a comparatively short distance from the surface the heat would be sufficient (at atmospheric pressure) to melt all kinds of mineral matter with which we are acquainted. It is supposed, however, by those who maintain that the earth is solid throughout, that the substance of the earth's interior is kept from liquefying by pressure. So far as geological facts go they are opposed to the view of a solid globe or of an enormously thick crust. The folding and contortion of strata seem to imply the presence of an underlying yielding mass upon which the solid crust may have a certain freedom to move during the shrinking and contraction that must result from the secular cooling of the earth (see EARTH, MOUNTAINS).

The origin of volcanic action has also been a much-canvassed question, and is variously explained according as the hypothesis of a solid or of a viscous interior is held to be the more probable (see VOLCANOES). Closely connected with such problems is that of the origin of oceanic basins and continental areas. Of late years the belief has gained ground that these dominant features of the earth's surface are of primeval antiquity—that in their origin they antedate the oldest of the sedimentary formations. It is a remarkable fact that hitherto, amongst the various formations that enter into the composition of the land of the globe, no trace of any abyssal accumulations has been met with. On the contrary, the aqueous rocks appear to have been deposited as a rule in relatively shallow seas. Many oscillations of level have taken place at successive periods within each continental area, by which the extent and outline of the land have been again and again modified, but the great continental ridges, according to the geological evidence, would appear to have persisted from the earliest times as dominant elevations of the earth's crust. 'The continents,' as Professor Dana remarks, 'have never changed places with the oceans.' See ABYSMAL ACCUMULATIONS.

Changes of Climate.—The geological record everywhere bears testimony to the fact that the climate of the globe has from time to time undergone changes. In our day climate is differentiated into zones; there is a marked change in the temperature as we pass from the equator to the poles. Latitude, and the relative positions of the great land and water areas, are doubtless the chief factors in the determination of the present climates of the globe, and must have had a similar influence on the climate of much older periods. Sir Charles Lyell and others have held, therefore, that such climatic vicissitudes as we have evidence of in the fossiliferous strata were probably induced by changes in the distribution of land and sea. Others have doubted whether this will explain the facts. If it be true that the great continental ridges are of primeval antiquity, then continents and seas could not have changed places, as Lyell supposed. The climatic conditions of the Glacial Period (q.v.) cannot possibly be due to such revolutions, for the distribution of land and sea during Pleistocene times was practically the same as at present. Stated briefly, the facts of geological climate are these: In Palæozoic ages the climate would appear to have been singularly genial and uniform over the globe. All through Mesozoic times similar genial conditions seem to have extended from what are now temperate up to polar regions. But the evidence indicates apparently that the climate of the latter was somewhat less genial than that of more southern latitudes. In Cainozoic ages, likewise, the climate continued to be mild even in high Arctic lands, but towards the close of the Tertiary era a general lowering of the temperature took place. Thereafter followed the Quaternary period with its extraordinary climatic changes (see GLACIAL PERIOD, PLEISTOCENE SYSTEM). It is possible, as some suppose, that the uniform climates of the earlier geological periods may have been due in part to the former greater heat of the earth. But probably the chief factor was the peculiar disposition of land and water. The continental areas appear for long ages to have been represented by groups of larger and smaller islands—a condition of things which would allow of the more or less free circulation of oceanic currents round the world. Under such conditions atmospheric temperature and pressure would have a very different distribution from the present. It can hardly be doubted, also, that cosmical causes must have had some influence upon former climates. Dr Croll believes that the strongly contrasted climates of the Pleistocene period (glacial and interglacial epochs) were the indirect result of increased eccentricity of the earth's orbit combined with the precession of the equinox. It has been objected to this theory that we have no evidence in the older geological periods of such remarkable climatic changes, which, if the theory be true, ought to have happened again and again during preceding periods of high eccentricity of the orbit. We are not, however, without evidence of ice-action in Palæozoic, Mesozoic, and Cainozoic times. The evidence is not abundant, but, considering the conditions of sedimentation, it is perhaps as much as could have been expected. It is doubtful, however, whether the arrangement of land and water in our hemisphere at any period anterior to later Cainozoic times could have favoured such enormous accumulations of snow and ice as those of the Pleistocene. When the continents were represented by groups of islands, the conditions for the massing of such great ice-fields could not have existed. And, if it be true that the climate of the globe in the earliest geological ages was influenced by the greater internal heat of the earth, the effects flowing from great eccentricity of the orbit might often be modified or neutralised.

Among the many subjects connected with geology which have separate articles assigned to them in this work, not to speak of the sections on the geology of Europe, Asia, Africa, America, Australia, and the several countries, are the following:

See, for General Geology, Lyell's Principles of Geology (1876); De la Beche's Geological Observer (1853); Lyell's Elements of Geology (1865); A. Geikie's Text-book of Geology (1887); Prestwich's Geology (2 vols. 1886-88); Phillips' Geology, edited by Etheridge and Seeley (2 vols. 1885); Green's Physical Geology (1882). The following are less elaborate treatises: Lyell's Student's Elements of Geology (1885); A. Geikie's Class-book of Geology (1886); J. Geikie's Outlines of Geology (1888); Jukes-Brown's Handbook of Geology (2 vols. 1884-86); Page and Lapworth, Introductory Text-book of Geology (1888). Of American and continental text-books may be mentioned: Dana's Manual of Geology (1875); Le Conte's Compend of Geology (1884); Credner's Elemente der Geologie (1887); Naumann's Lehrbuch der Geognosie (3 vols. 1858-72); Allgemeine Erdkunde, by Hann, Von Hochstetter, and Pokorny (1881); De Lapparent's Traité de Géologie (1884); Stoppiani's Corso di Geologia (1871). Hutton's Theory of the Earth (1795) is interesting as containing the groundwork of the modern system of geology. See also Playfair's Illustrations of the Huttonian Theory (1822). Of works dealing with special branches of geology the following may be cited: For Cosmical Aspects of Geology, see Sir W. Thomson, 'On the Age of the Sun's Heat,' in Popular Lectures and Addresses (vol. i. 1889); Croll's Climate and Time (1875), Climate and Cosmology (1885), and Stellar Evolution (1889). For Petrographical Geology, see references under PETROGRAPHY. For Dynamical Geology, see Darwin's Geological Observations on Volcanic Islands (1884), and Observations on South America (1846; both works in 1 vol. 1876); Scrope's Volcanoes of Central France (1858), and Volcanoes (1872); Judd's Volcanoes (1881); R. and J. W. Mallet's Earthquake Catalogue (1858); Milne's Earthquakes (1886); Fuchs's Vulcane und Erdbeben (1875); Fouqué's Les Tremblements de Terre (1888); Twelfth Annual Report of U.S. Geological and Geographical Survey of the Territories (1883; for Geyzers); Fisher's Physics of the Earth's Crust (1882); T. G. Bonney, The Story of our Planet (1894); Bischoff's Chemical and Physical Geology (1854-59), and the Supplement (in German, 1871); Roth's Allgemeine und chemische Geologie (1879); Agassiz' Études sur les Glaciers (1840); Forbes's Travels through the Alps (1843), and Papers on the Theory of Glaciers (1849); Tyndall, The Glaciers of the Alps (1857); Darwin's Vegetable Mould and Earthworms (1881), and Coral Reefs (1874); Dana's Coral and Coral Islands (1875). Further references to special works dealing with dynamical geology will be found in the larger text-books of geology. For Structural or Geotectonic Geology, consult the standard text-books of geology; see also article STRATUM. For Experimental Geology, see Daubrée's Études Synthétiques de Géologie Expérimentale (1879). For works dealing with Palæontology, see under that article. For Physiographical Geology, see Memoirs of Geological Surveys of British Islands, passim; Ramsay's Physical Geography and Geology of Great Britain (1878); A. Geikie's Scenery and Geology of Scotland (1889); Hull's Physical Geography and Geology of Ireland (1878); Dutton's 'Tertiary History of the Grand Cañon District,' Monographs of U.S. Geol. Survey (vol. ii. 1882); also Annual Reports of U.S. Geol. and Geograph. Survey of Territories (1867-78), passim; De la Noë and De Margerie, Les Formes du Terrain (1888). For Geology of British Islands, see Maps and Memoirs of the Geological Survey; works by Ramsay, A. Geikie, and Hull already cited; Woodward's Geology of England and Wales (1887); Kinahan's Geology of Ireland (1878); Murchison's Siluria (1867); Macculloch's Western Islands of Scotland (1819); Nicol's Guide to the Geology of Scotland (1844)—these last two works rather out of date; Miller's Old Red Sandstone (1858); Green, Miall, and others, Coal: its History and Uses (1878); Hull's Coalfields of Great Britain (1881); Meade's Coal and Iron Industries of the United Kingdom (1882); Phillips' Geology of Oxford and the Valley of the Thames (1871), and Geology of the Yorkshire Coast (1875); Tate and Blake, The Yorkshire Lias (1876). For further references to treatises dealing with the geology of England and Wales, see especially Woodward's work cited above. The following works deal with Pleistocene Geology and the Antiquity of Man: Lyell's Antiquity of Man (1873); Lubbock's Prehistoric Times (1878); Evans' Ancient Stone Implements of Great Britain (1872); Dawkins' Cave-hunting (1874), and Early Man in Britain (1880); J. Geikie's Great Ice Age (1877), and Prehistoric Europe (1881); Dawson, The Earth and Man (1887); De Quatrefages, The Human Species (1879); Joly's Man before Metals (1883); Penck's Die Vergletscherung der deutschen Alpen (1882); Falsan, La Période Glaciaire (1889); Wright's Ice Age in North America, &c. (1889). For treatises bearing on Geological Climate, see Croll's works already cited; also J. D. Whitney, The Climatic Changes of Later Geological Times (1882). Amongst works on Economic Geology the following may be mentioned: Page's Economic Geology (1874); Williams' Applied Geology (1886); Penning's Engineering Geology (1880); Nivoit's Géologie appliquée à l'Art de l'Ingénieur (1887). For methods of geological observation and the making of geological maps, see the larger text-books, Sir A. Geikie's Outlines of Field Geology (1879), and Penning's Field Geology (1876). Sir A. Geikie, The Founders of Geology (1897), deals with Desmarest, Guettard, and other early geologists.

Geomancy. See DIVINATION.