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It has been suggested that such highly felsitic and acid lavas as that of which the Puy de Dôme, the Grand Sarcoui, and Cliersou are composed, may have had their origin in the granite itself, melted and rendered viscous by intense heat. Dr.
Taking the Puy de Dôme, Petit Suchet, Cliersou, Grand Sarcoui in Auvergne, and the Mamelon in the Isle of Bourbon as illustrations, we have in all these cases a group of volcanic hills, dome-shaped and destitute of craters, the summits being rounded or slightly flattened.
On the other hand, lavas less fusible, or poured out at a lower temperature, stiffen when they have flowed but a short distance, and accumulate in a steep cone. Trachyte has been extruded in a state so viscid that it has formed steepsided domes like that of Sarcoui.
The same may be said of Le Grand Sarcoui, shaped like a kettle turned bottom upwards. In some of the galleries are unfinished sarcophagi. But although originally quarries, they were used as refuges in later times. At Corent, on the Allier near Veyre-Mouton, are refuges in caves, so also at Blot-l'Eglise near Menat, which served the purpose during the troubles of the League.
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