ITAL Y The southern or Italian slope of the Alps differs in several respects from the northern. The descent is much more abrupt, and in the west the outer rocks are entirely wanting. Further east the lime stone appears, but nowhere is it so fully developed as on the northern slope. Many of the rivers are transverse as they issue on to the plain, but within the mountains there are numerous longitudinal valleys, of which that of the Adda is the most important. The morainal material brought down by Alpine glaciers has built up between the mountains and the plains a belt of hills, which in many cases has dammed the river courses and caused the formation of lakes.
The plains of the Po were formerly occupied by a branch of the Adriatic, which gave place to solid land, partly as a result of the deposition in it of the debris carried down by Alpine and Apennine glaciers and streams, and partly as a result of the upheaval of its bed. The surface of the region is thus composed of Quaternary material and is generally level. Even at the present time the work of the rivers in building up the plain has not ceased, and around the mouths of the Po the land is gaining at the expense of the sea.
The Ligurian Alps are considered to end, and the Apennines to begin, at the Collo dell' Altare, west of Savona. From that pass the Apennines pursue their course throughout the whole length of the peninsula, crossing over to the east coast at Ancona, and returning to the west in Calabria. They do not form a simple anticlinal fold, but consist of a series of folded ranges. In Liguria and Emilia in the north, where they are built up of clays and other material of Tertiary age, the general trend of the ranges is to the south-east, and each fold in turn loses its height towards that direction, while its function as a watershed is taken over by the range lying immediately to the east. On the west the rivers are generally longitudinal, flowing in synclinal valleys, and thus facili tating communications, while on the east they are, as a rule, trans verse. The Central Apennines of Umbria and the Marches in the north, and the Abruzzi in the south, also exhibit a folded formation, but in a less marked degree than in the north. The rocks of which they consist are largely of Jurassic and Cretaceous origin, and the scenery is bolder and wilder than it is on the Tertiary clays, while in places the characteristics of a Karst region prevail. The southern Apennines are distinguished rather by their broken and irregular features than by folding. They fall into two divisions : the Nea politan and the Calabrian. The former are built up of Triassic and Tertiary rocks, while the latter consist in part of Tertiary rocks, and in part of the mountainous regions of Sila and Aspro monte, fragments of the ancient Archaean land mass of Tyrrhenia.
Of this land mass, Sardinia and parts of Sicily, Elba, Tuscany, and Calabria are now the only remains.
On the west coast, between the Apennines and the Tyrrhenian Sea, there is a region of comparatively low elevation, built up in various ways. In Tuscany, in the north, are fragments of the Tyrrhenian block already mentioned. Further south, along the zone of fracture, there are volcanic districts such as the Alban Hills, and the Phlegrman Fields with the active cone of Vesuvius. The plains of Rome and Campania consist in the main of volcanic debris, which was originally deposited on the floor of the ocean, and subsequently raised above sea level ; but the lower basins of the Arno, the Tiber, and some smaller rivers, are covered with alluvium brought down from the Apennines.
On the east coast, there extends from the river Fortore to the Gulf of Taranto the province of Apulia, a low-lying plateau with an average height of about 600 feet. Tertiary rocks prevail except in the Murgie where Cretaceous formations, frequently covered with loess, reach an elevation of over 2,000 feet.
The climate of Italy presents several features of interest. The temperature is affected by the modifying influence of the sea, and by the protection from cold northerly winds afforded by the Alps, and in a less degree by the Apennines. In the Alps, of course, temperature decreases with altitude, but many of the sheltered valleys have mild winters ; milder, indeed, than those of the plains. The latter, being cut off from the westerly winds by the surround ing mountains, have a range between summer and winter almost continental in character ; the average temperature for January being about 34° F., and for July about 74° F. In peninsular Italy, where the influence of the sea is felt to a greater extent, the January temperature is higher and varies from 40° F. in the interior and about 41° F. on the Adriatic to about 45° F. on the Tyrrhenian coast, while the average July temperature over the whole region is from 74° F. to 76° F.
The rainfall occurs mainly in the winter half of the year. In South Italy the summers are rainless, but further north the dis tinction between summer and winter is less marked, and in the northern parts of the plains of the Po the rainfall is fairly evenly distributed throughout the year. On the Alps and on the Alpine borderland the annual precipitation varies from 40 to 60 inches, while on the Ligurian coast, on the Northern Apennines, and over a district lying to the south of Rome, it exceeds 40 inches. Else where it varies from 30 to 40 inches, except in Apulia and Sardinia, where it is generally between 20 and 30 inches.