The disorder and ruin of these several points of the original system, and the almost total destruction of its northern part, were undoubtedly caused by the third and last upheaval which gave the island the form it presents at the present day. Its direction was from north to south, and so long as the mass then raised did not come in contact with the land created by former upheavals, it preserved its regular line, as we find in the mountain-chain of Capo Corso. But when, on emerging above the surface of the sea, this mass had to overcome at its southern extremity the resistance of the primary rocks upheaved long before, and now become hard and consolidated,—in that terrible shock, on the one hand, it changed, crushed, or ruined all that obstructed its progress, while, on the other, it varied its own direction and was itself broken up in many places, as appears from the openings of the valleys communicating from the interior with the plains of the eastern littoral and giving a passage to the torrents which fall into the sea on this coast,—the Bevinco, the Golo, the Tavignano, the Fiumorbo.
The fundamental rocks brought up by this third and last upheaval are ophiolitic, and metamorphic, or primary, limestone, overlaid in some places by secondary formations. “The granites on the west, as well as the south, of the island include some beds of gneiss and schistes at their extremities.”—(Gueymard). Almost everywhere the granite is covered—an evident proof that the epoch of its eruption preceded that when the deposits were formed in the depths of the sea, and deposited in horizontal strata on the crystalline masses of the granite.
Masses of euritic and porphyritic rocks intersect the granites, and a distinct formation of porphyries crowns Monte Cinto, Vagliorba, and Pertusato, the highest summits of the Niolo, covering the granite. These porphyries are pierced by greenstone two or three feet thick, and the granites are intersected by numerous veins of amphibolite (hornblende) and greenstone, generally running from east to west.
Transition rocks, as they are called, occupy the whole of Capo Corso and the east of the island. They consist of talcose-schiste, bluish-grey limestone, talc in beds, serpentine, black marble similar to the oldest in the Alps, quartz, feldspar, and porphyries.
The tertiary strata are only found at certain points in isolated fragments. One of these occupies the bottom of the Gulf of San Fiorenzo and part of its eastern shore. There the beds rest with a strong inclination against the lower declivities of the chain of Capo Corso, rising from upwards of 600 to 900 feet above the level of the Mediterranean,—a distinct proof that their formation at the bottom of the sea was anterior to the upheaval of that chain, and of the whole system of mountains having their direction north and south.
In the deep escarped valleys between San Fiorenzo and the tower of Farinole, the tertiary deposits are seen in successive layers forming beds which in some places are in the aggregate from 400 to 500 feet thick, and the calcareous beds contain great quantities of fossil remains of marine animals of low organisation, such as sea-urchins, pectens, and other shells; forming a compact mass, of which the greater part of the formation consists. The singular phenomenon of the presence of rounded boulders of euritic porphyry, resembling that of the Niolo, embedded in these strata, proves to a certainty that at an epoch anterior to the upheaval of the system running north and south, and of the mountains of La Tenda depending on it, the high valleys of the present bason of the Golo, and especially that of the Golo, were prolonged to the sea.
A second tertiary deposit exists near Volpajola, on the left bank of the Golo, nearly eight miles from the eastern coast. The beds lying horizontally are full of shells.
We find a third fragment of a tertiary formation on the part of the littorale stretching from the mouth of the Alistro to that of the Fiumorbo, in the middle of which stood the ancient city of Aleria. In some places these beds have been lifted without any sensible alteration of their original form of deposit in horizontal strata, and throughout they bear a close resemblance to the tertiary formation of San Fiorenzo.
A fourth, and more striking, example of the same formation is exhibited at the southern extremity of the island. There we find an horizontal plateau from 200 to 300 feet high between the Gulf of Sta-Manza and Bonifacio. The promontory on which that town and fortress stands, and the whole adjoining coast along the straits, present exactly the same appearances as the white chalk cliffs of Dover; and at the Cala di Canetta these calcareous rocks rise à pic over the sea 150 and 200 feet. There is a perfect analogy between this formation and those of San Fiorenzo and the Fiumorbo already mentioned. Only, this last contains a much greater variety of fossil remains, both animal and vegetable, consisting of lignites, oyster-shells, large pectens, operculites, and fragments of sea-urchins, polypi, &c. We shall have an opportunity of mentioning hereafter the curious caverns worn in the soft calcareous rock by the force of the waves lashing this coast with so much violence in the storms to which the Straits of Bonifacio are exposed.
Coming now to the alluvial deposits, we find them extending over the great plains on the eastern coast of the island, the littorale mentioned in an early chapter of this work. The plain of Biguglia, for instance, was formed by one of those vast inundations which have received the name of diluvial currents, and swept away a great number of species of animals. In fact, we find traces of one of these inundations in a breccia formed of the fossil bones of animals in the hills near Bastia. Among these fossil bones Cuvier has remarked the head of a lagomys, a little hare without any tail,—a species still existing in Siberia.[22] It would too much lengthen these remarks were we to enter on an inquiry into the age and character of these osseous breccia, but the curious reader is referred to Lyell's “Elements”[23] for some interesting observations on fossil mammalia found in alluvial deposits alternating with breccia. We are not aware, however, that the hills near Bastia are connected with volcanic action as those of Auvergne, to which Mr. Lyell refers.