Cambrian System.

Cambrian System. This is the name given to the great series of sedimentary deposits which comes next in order of succession to the Archæan System (q.v.). Wherever the base of these deposits can be seen, the beds are found to rest unconformably upon the Archæan, or as they are sometimes termed, the pre-Cambrian rocks. There is some diversity of opinion as to the upper limits of the Cambrian. The name was introduced by Professor Sedgwick, by whom it was made to include a great thickness of strata which most geologists now prefer to consider as forming the lower portion of the succeeding Silurian System (q.v.). According to Dr Hicks, whose classification is now generally adopted for the British Cambrian, the divisions of this system are as follows:

The British Cambrian rocks are best developed in North Wales, and are also well represented in Shropshire, &c. They consist largely of coarse red and purple graywackes, sandstones, grits, and conglomerates, and grayish blue and green slates and slaty shales; and the same general character is maintained by the rocks of this system in other parts of the world. In Ross-shire and adjoining districts in the north-west of Scotland, certain dark reddish-brown conglomerates and sandstones are found overlying unconformably the Archæan rocks of that region, and are themselves covered unconformably by Silurian strata. They form the pyramidal-shaped mountains of Suilven, Canisp, and Coulmor. Cambrian strata with a few fossils have also been recognised in the south-east of Ireland, where they attain a thickness of 14,000 feet at least. The Cambrian strata are for the most part unfossiliferous—organic remains being met with chiefly in the higher members of the system. The plant-life of the period is very sparingly, if at all, represented, for it seems doubtful if the fucoid-like markings are really traces of plants. Some of these are almost certainly inorganic—mere wrinkles on the surfaces of the beds, while others are not improbably the tracks or trails of worms, crustaceans, &c. One of the forms which has been banded about between the vegetable and animal kingdoms is Oldhamia, some holding it to be a calcareous seaweed, others ranking it with the Sertularida, or even the Polyzoa. Animal life, however, is surprisingly well represented, as by sponges (Protospongia), sea-lilies (Dendrocrinus), cystideans (Protocystites), and star-fishes (Palæasterina). Worm-burrows and worm-castings often abound, and crustaceans are plentiful—the modern groups of Ostracods (water-fleas) and Phyllopods (brine-shrimps, &c.) being represented. The most notable crustaceans, however, are the Trilobites (q.v.), some of which were very minute and blind (Agnostus), while others attained a length of one or two feet (Paradoxides). The Brachiopods belong almost exclusively to the ‘inarticulate’ group—the three most characteristic forms being Lingulella, Discina, and Obolella. Four out of the five classes of Molluscs now existing appear in the Cambrian—viz. lamellibranchs, pteropods, gasteropods, and tetrabranchiate cephalopods. The last are represented by straight and curved chambered shells (Orthoceras, Cyrtoceras) which belong to the family of the Nautilidae. The lamellibranchs and gasteropods, which attain their maximum in our own day, appear to have been but sparsely present in the seas of Cambrian times, but some of the gasteropods, such as the snail Pleurotomaria, are types still living. The same, it may be remarked, is the case with some of the brachiopods (Lingulella, Discina), which have persisted to the present day. No indubitable traces of vertebrate life have yet been supplied by the Cambrian strata, nor have we any evidence of land or fresh-water forms, although it is quite possible that some of the molluscs may have lived in estuaries or brackish water. The general character of the sedimentary rocks, with their conglomerates, grits, &c., ripple marks, and animal tracks, is indicative upon the whole of shallow-water conditions; and the general absence of corals and of limestones points in the same direction. Although the fauna of the Cambrian is less varied and abundant than that which has left its traces in the succeeding Silurian, yet we cannot but be struck with the fact that the former is relatively so varied. For here in the very oldest fossiliferous strata (if the Archæan with its Eozoon be for the moment put out of account) we have most of the larger divisions of the animal kingdom represented. Hence those who believe in the doctrine of evolution are of opinion that the Cambrian fauna cannot possibly be the earliest, but must have been preceded by a long series of ancestors; and not only so, but that many forms of life must have existed in the Cambrian seas which have left no trace behind them. Do fossiliferous strata older than the Cambrian remain to be yet discovered? or is it possible that the Archæan schists represent such strata from which all traces of organic remains disappeared during the process of metamorphism? To such questions no conclusive answer can be given at present.

Cambrian rocks have been recognised in various other parts of Europe, as in Central and Southern Sweden, where the strata are not nearly so thick as in the British area. In Central Brittany and in the Ardennes they are likewise represented, and they also come to the surface in several provinces of Spain. The most important continental area, however, is that of Bohemia, where the palæozoic rocks and their fossils have been studied in great detail by M. Barrande. He divides the strata into several zones, his primordial zone having yielded many organic remains, especially trilobites belonging to characteristic Cambrian genera. In North America the Cambrian or Primordial system comprises an upper series of shales and sandstones (Acanadian series), and a lower one of sandstones, &c. (Potsdam series). These strata have been recognised in Newfoundland, Nova Scotia, New Brunswick, and Canada, and in the states of Massachusetts, Vermont, and New York. They appear also in many different places along the Appalachian chain and in the region of the Rocky Mountains.

The Cambrian rocks of Europe are frequently much altered and metamorphosed, while in North America they show over certain wide areas comparatively little alteration—the Potsdam sandstone especially being not unlike many ‘freestones,’ so that it is much used for building purposes. Igneous rocks (diorite, diabase, quartz-porphyry, &c.) are associated with the Cambrian in various parts of Europe. Not much can be said as to the physical and climatic conditions of the Cambrian period. The strata, as we have seen, point generally to shallow seas. The main land-surfaces in the European area probably lay towards the north and north-west of the regions now occupied by Cambrian strata. But large islands of Archæan rocks may have existed in Middle Europe. According to Sir A. C. Ramsay, the red unfossiliferous conglomerates and sandstones may have been accumulated in inland seas or lakes. This he thinks was the origin of the red sandstones in the north-west of Scotland. And he has suggested that some of the conglomerates of the same district may be of glacial origin. The general character of the Cambrian fauna does not justify us in forming any conclusion as to the climatic conditions of the period. All we can say is that there is no evidence to show that the climate of the globe was then differentiated into distinct zones as it is at present.