374 SPECIAL GEOLOGY. 



being twenty miles, and its length between forty and fifty. 

 The surface is formed of pebbles and boulders of granite 

 and basalt, reposing on limestone ; the river Allier flows 

 through the district, over beds of this limestone, or siliceous 

 sandstone, except where it has occasionally excavated a 

 channel to the granitic foundation-rock. Hills composed 

 of calcareous and alluvial deposits, which are scattered over 

 the district, are supposed to be the relics of a more ancient 

 plain, once existing at a higher elevation than the present, 

 some of them being surmounted with basalt, others with 

 limestone, which has protected and preserved them. On the 

 western side of the plain the limestone disappears, and a 

 plateau of granite rises 1600 feet above the valley of Cler- 

 mont. This mass of granite supports a series of not less 

 than seventy volcanic cones, varying in height from five 

 hundred to a thousand feet, and forming a range nearly 

 twenty miles in length, and two in breadth. The most 

 remarkable of these ancient vents, are the Puy de Come 

 and the Puy de Montgy. 



For a description of these in detail, as well as of the 

 neighbouring volcanoes of the Vivarais, and the Cantal, we 

 must refer to Mr. Scrope'sw r ork, and shall content ourselves 

 with calling attention to one or two of the most remarkable 

 features of this interesting district.* 



Among the agencies of nature, volcanic action is one of 

 the most awful and powerful. A force which melts up the 

 solid materials of the earth, and ejects them as floods of 

 lava over the surrounding districts, which lays waste the 

 most fertile regions, overwhelms the fairest cities, and 

 entombs them in its ashes, naturally arrests our attention. 

 This tremendous' power was most extensively developed in 

 this region. "When we contemplate not a single volcanic 

 vent, but numbers of these comprised in a limited area, we 

 recognise the vast scale on which this agency was exerted 

 during the tertiary period. Having thus briefly alluded to 

 these mightier manifestations of creative power, we shall 

 now direct the attention of the student to some phenomena 

 in the same region regarded as the most insignificant. 



There is a little crustacean (Ch/pris), some species of which 



* Sir C. Lyell's Principles of Geology contains a valuable epitome of the 

 volcanic phenomena of Auvergne. 



