Apennines (Ital. Appennini, Lat. Mons Apenninus; from Celtic pen, 'a head' or 'mountain-height'), a mountain-chain extending 740 miles uninterruptedly throughout the whole length of the Italian peninsula. It belongs to the system of the Alps, from which it branches off near Savona. From this point, the chain, under the name of the Ligurian Apennines, girdles the Gulf of Genoa, in the immediate vicinity of the sea, and then runs inland, forming the watershed between the Adriatic and the Mediterranean, but gradually approaching the east coast, till, in the highlands of the Abruzzi, it is close upon it; after which it takes a southerly direction through Naples, dips under the sea at the Strait of Messina, and reappears on the northern coast of Sicily. The chain is divided by geographers into the Northern, the Central, the Southern, and the Sicilian Apennines. The leading feature of the Apennines, wherever they approach the coast, is their extraordinarily steep declivities; while in Middle Italy and the adjoining portions of Upper and Lower Italy, long terraced plateaus, lower ranges, and extensive coast-plains mark their gradual descent on the west. The main chain of the Apennines does not send off spurs into the Apulian peninsula.
The direction of the great chain of the Apennines is favourable to the formation, on the west side, of largish river-basins, such as those of the Arno, the Tiber, the Garigliano, and the Volturno; while on the east side we find nothing but small streams, in most cases destitute of affluents, hurrying down to the sea through wild precipitous valleys.
The average height of the entire chain of the Apennines is about 4000 feet, which, however, in the north, sinks down to little more than 3500 feet; and in the mountains of the Abruzzi, rises to 7000 feet. Here, in Monte Corno, the highest peak of the range known under the name of Gran Sasso d'Italia, they reach an elevation of 9574 feet, and in Monte Velino, of 7916 feet.
The Apennines are crossed by thirteen principal passes; and seven of these are traversed by the railway. The strata which compose the Apennines are principally of Jurassic, Cretaceous, and Tertiary age. The central ridges, especially in the middle and south, consist largely of Jurassic limestones, &c. These are flanked on the north-east and south-west by more or less interrupted belts of Cretaceous rocks, which again are overlaid by Pliocene formations. In the north, rocks of Eocene and Miocene age predominate. The Apennines, especially the Roman and Neapolitan, are distinguished by the rich variety of marbles which they contain. Volcanic rocks abound in the middle and southern regions.
The principal chain exhibits, for the most part, a dreary and barren appearance. It looks like a vast wall, with very few projecting peaks to break its dull monotony. Only in the Abruzzi, in the Sub-Apennines, and above all, in the marble mountains of Carrara and Seravezza, do the bold and magnificent forms of the Alps appear. Where water is plentiful there is no lack of rich pastures and dense forests; but usually only thin grass and wild scrubby bushes cover the stony slopes. The greater number of the forest brooks, with deep rocky ravines, display, during summer, only a dry bed. Where the mountains dip down to the sea, as at the Riviera of Genoa and the Gulf of Naples, a rich, peculiarly southern vegetation clothes the declivities. There is no region of perpetual snow; but the summits of the Abruzzi and the lofty peaks of Lunigiana are often covered with snow from October far into May, and send their icy breath so suddenly down into the mild valleys, that the temperature in a few hours sinks 12° or 18° F.