Page:A book of the Cevennes (-1907-).djvu/27

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7
VOLCANIC VENTS

chain of the Boutières, composed of granite and gneiss, and they are the least interesting portion of the series. Few of the summits surpass 3,600 feet, but they throw out a spur that is a supreme effort, the Mont Pilat, 4,700 feet; precisely as the Pyrenees, before expiring in the east, have projected to the north-east, and tossed aloft the noble pyramid of the Canigou. The Boutières attach themselves at their southern extremity to Mézenc, the loftiest peak of the Cevennes, 5,750 feet. So also does the chain of the Mégal, separated from the Boutières by the valley of the Lignon. Seen from Le Puy, this ridge is fine, broken into peaks. The Mégal itself attains to the height of 4,345 feet. This cone formerly belched forth a torrent of lava reaching to a thickness of 450 feet, and extending to a distance of fifty miles by eight miles wide.

From the volcanic nucleus of Mézenc branches southeast the chain of the Coiron, volcanic as well, stretching to the Rhone, where its last deposits of lava are crowned by the ruins of Rochemaure. The geologist Cordier, who had travelled in Auvergne, Italy, Syria, and Egypt, declared that he had never seen a volcanic region comparable to the Coiron. This chain is of special interest to the geologist, and is full of surprises to the ordinary traveller, for the lava bed caps the mountains, composed of friable limestone, that once formed a great calcareous plain. The Rhone has lowered its bed a thousand feet since the liquid stone flowed, and torrents have cut through lava and limestone, fashioning deep and even broad valleys. Next, the weather ate into the flanks where the stone was soft, undermined the basalt, that came down for lack of support in huge masses.