While the principal cities and towns in Corsica stand within the limits of the first zone, it is in the second that by far the greatest part of the population live,—dispersed, as we have often had occasion to remark, in valleys and hamlets placed on the summits or ridges of hills. The choice of such positions is a necessary condition of health, as in this region, no less than in the former, the valleys are notorious for the insalubrity of the air.
The third zone, ranging from an elevation of about 6000 feet to the summits of the highest mountains, is a region of storms and tempests during eight months of the year; but during the short summer the air is said to be generally serene, and the sky unclouded. This elevated region has, of course, no settled inhabitants, but during the fine season the shepherds occupy cabins on its verge, their sheep and goats browsing among the dwarf bushes on the mountain sides. The vegetation is scanty. Even the pine cannot thrive at such an elevation, and the birch, which one generally finds, though dwarf, still higher up the mountains, I did not happen to see in Corsica, though it is mentioned in Marmocchi's list of indigenous trees.
The summits of the Monte Rotondo and Monte d'Oro are capped with snow at all seasons, and beautiful are snowy peaks, piercing the blue heavens in the sunny region of the Mediterranean, and well does the glistening tiara, marking from afar their pre-eminence among the countless domes and peaks which cluster round them, or break the outline of a long chain, assist the eye in computing their relative heights. We had no opportunity of ascertaining how low perpetual snow hangs on the sides of the highest Corsican mountains. According to M. Arago, Monte Rotondo is 2762 mètres (about 8976 feet) above the level of the sea; and he says that there are seven others exceeding 2000 mètres (about 6500 feet). Among these must be included Monte d'Oro, which figures in Marmocchi's list at 2653 mètres, or about 8622 feet. The season was too late for our making an ascent with any prospect of advantage; but at that time of the year (the end of October) none of the peaks we saw, except the two named, though some of them are only from 500 to 800 feet lower than Monte d'Oro, had snow upon them.
While rounding the base of Monte d'Oro, we observed long streaks on the side of the cone, descending, perhaps, 1000 feet below the compact mass on the summit; but they had the appearance of fresh-fallen snow, and from our observing that all the other summits were free from snow, I am inclined to assign the height of about 7500 or 8000 feet above the level of the Mediterranean as the line of perpetual snow in Corsica.
In Norway, between 59°-62° N. latitude, we calculated it at about 4500 feet on the average, the line varying considerably in different seasons. In the summer of 1849 there was snow on the shores of the Miös-Vand, which are under 3000 feet, while the summer before the lakes on the table-land of the Hardanger Fjeld, 4000 feet high, were free from ice, and throughout the passage of the Fjeld the surface covered with snow was less than that which was bare. In 1849, crossing the Hardanger from Vinje to Odde, the whole of the plateau was a continued field of snow.[15] Taking the entire mountain system of central Norway, from the Gousta-Fjeld to Sneehættan and the Hörungurne, with elevations of from 5000 to near 8000 feet, the average of the snow-level may be taken, as before observed, at about 4500 feet; that of the Corsican mountains, with elevations of from 6000 to nearly 9000 feet, being, as we have seen, from 7000 to 8000 feet.
In Switzerland, where the elevations are so much greater, the snow-line varies from 8000 to 8800 feet above the level of the sea.[16] On Mont Blanc it is stated to be 8500 feet. The height differs on the northern and southern faces of the chain within those portions of the Alps that run east and west, but 8500 feet may be taken as the average.
We may be surprised to find that congelation rests at the same, or nearly the same, level in the Alps of Switzerland, and on the Corsican mountains eight degrees further south. But difference of latitude is no determinate rule for calculating the level to which the line of perpetual snow descends. There are other influences to be taken into the account, such as the duration and intensity of summer heats, the comparative dryness of climate, the extent of the snow-clad surface in the system generally, and more especially the height and exposure of particular mountains.[17] Thus the snow-line on the southern slope of the Alps is in some cases as high as 9500 feet. It may be conceived that as the great extent of snow-clad surface on the high Fjelds of Norway so much depresses the level of the snow-line in that country, so the great superincumbent mass resting on the summits of the higher Alps has a similar effect, reducing the average snow-line in Switzerland to nearly that of the Corsican mountains. The wonder is that Monte Rotondo and Monte d'Oro,—rising from a chain surrounded by the Mediterranean, in insulated peaks of no very considerable height, without glaciers or snowy basins to reduce the temperature,—should, in a climate where the sun's heat is excessive for eight months of the year, have snow on their summits in the months of July and August. I have observed the Pico di Teyde in Teneriffe with no snow upon it in the first days of November, though it is 3000 feet higher than Monte Rotondo, and only five degrees further south. Mount Ætna, also, nearly 11,000 feet high, in about the same latitude as the Peak of Teneriffe (37° N.), is free from perpetual snow; but that may arise from local causes.