VOLCANOES OF CENTRAL FRANCE. 



269 



lariy interesting to the geologist. In Auvergne, and the more sotitli- 

 em parts of France, there are extinct volcanoes of different ages, 

 c©vering with their products several thousand square miles. The 

 most recent of these volcanoes has been extinct or dormant since 

 ihe records of authentic history, and probably for a longer period. 

 Julius Caesar, who was encamped on this volcanic soil, and has de- 

 scribed the country, makes no allusion to its having been the seat of 

 active volcanoes.* 



West of the town of Clermont, there is an extensive granitic plain, 

 rising about sixteen hundred feet above the level of the river Allier. 

 On this plain there are numerous cones, and dome-shaped hills, va- 

 rying in height from twelve hundred to two thousand feet ; some of 

 these cones have well-preserved craters, and the cones themselves 

 are formed chiefly of scoriaceous lava. These are the most recent 

 volcanoes of that country ; their products differ in no respect from 

 those of modern volcanoes, except that the lava may often be ob- 

 served passing to the state of compact basalt, exactly similar to many 

 of the basaltic rocks in Great Britain. That these volcanoes are the 

 most recent, is proved by the lava flowing down from them into the 

 present valleys ; and hence we are certain, that the eruptions must 

 have taken place subsequently to the excavation of the valleys. 

 There are other currents of lava from more ancient volcanoes, that 

 have flowed before the valleys were excavated, and form isolated 

 caps on the hills that enclose the present valleys. These currents 

 of lava are composed chiefly of compact basalt : the position of these 

 isolated caps of basalt is similar to that on the hill 6, (Plate III. fig. 

 2.) but they are not always columnar. The openings from which 

 these beds of basalt have flowed cannot be always traced ; but as we 

 can observe the change from scoriaceous lava to basalt in the cur- 

 rents of undoubted lava, we cannot hesitate to admit, that the basalt 

 which forms these caps, must have had a similar origin. Under the 

 caps of basalt, there are in many situations thick beds of volcanic 

 tufa, containing bitumen, which will be subsequently noticed. Beside 

 the volcanoes with craters, that have ejected currents of scoriaceous 

 lava and basalt, and poured them into the valleys ; and beside the 

 more ancient volcanoes, that have formed beds of basalt before the 

 excavation of the valleys, — there are other volcanic mountains, which 

 have rounded summits or domes, without any perforation or crater, 

 and these are composed chiefly of whitish or grey earthy felspar, 

 containing imbedded crystals of felspar : to this rock the name of 

 trachyte has been given, on account of its rough fracture. It may 

 be properly called a volcanic porphyry. 



* I visited the extinct volcanoes of France in the spring of 1822, and published 

 an account of them in the 2d volume of my Travels, accompanied with cuts, and 

 a section and outline of the country round Clermont, which is, I believe, the first 

 attempt to render in this manner the structure of this volcanic district intelligible 

 to the general reader. Without the aid of sections and diagrams, it is difficult to 

 obtain a distinct notion of the relative position of the different volcanic formations. 



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