Between this long chain, extending from Monte Antonio to Monte Incudine, and the tortuous ranges detached obliquely from it, lies a central area equal in surface to a fifth part of the whole island of which it forms the heart—the interior. The general inclination of this area, with the openings of the valleys, tends to the east. It does not form one single bason, but, intersected as it is in various directions by secondary ranges, and by mountains linking the principal chain, its _contour_ is composed of a series of deep and generally narrow valleys, rising one above the other. The grandest as well as the most elevated of these basons is that of the _Niolo_, the citadel of Corsica.
These lofty mountain chains, with the numerous ramifications detached from them, and extending in all directions, render the communications between one place and another, between the coasts on opposite sides of the island, extremely difficult. The passage from the western to the eastern shore can only be effected by climbing to great elevations, through long and narrow gorges, through deep ravines of savage aspect, and covered with dense forests. The Corsicans give a lively idea of some of these toilsome paths by calling them _scale_,—ladders, staircases;—and such, indeed, they are, the steps, often prolonged for miles, being partly the work of Nature, partly cut in the rock by the hand of man.
In the present state of science there can be no difficulty in ascribing the origin of the three great lines of the Corsican mountains, to which all the others are subordinate, to three vast upheavings of the soil in the direction they take. The order of these elevations above the surface of the ancient sea thrice repeated in the long series of past ages, giving the first existence to the island, and by successive conglomerations shaping its present bold and irregular profile, may be also distinctly traced.
The masses first raised to the surface of the sea, supposed to be of igneous origin, lifted by the intense action of fire or subterranean heat from vast depths, and called by English geologists “Plutonic rocks,” as differing from “Volcanic,”—these masses constitute nearly the whole south-western coast of Corsica, one half of the whole island.