268 THE WONDERS OF GEOLOGY. Lect. III. 



47. Extinct volcanoes of Auvergne. (Plate II.) — 

 The country which is the site of these extinct volcanoes is 

 about 220 miles south of Paris, and forms a vast plain, 

 situated in the department of the Limagne d' Auvergne. 

 It is so remarkable for its fertility, that it is called the 

 Garden of France ; a quality attributable to the detritus of 

 the volcanic rocks, which enters into the composition of the 

 soil. It is inclosed on the east and west by two parallel 

 ranges of gneiss and granite. Its average breadth is 

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

 altitude about 1,200 feet above the level of the sea. The 

 surface of this plain is formed of alluvial deposits, com- 

 posed of granitic and basaltic pebbles, and boulders, re- 

 posing on a substratum of limestone. Hills, of various 

 elevations, composed of calcareous rocks, are scattered 

 over the plain ; and the river Allier flows through the 

 district, over beds of limestone and sandstone, except where 

 it has excavated a channel to the foundation -rock of 

 granite. The hills formed of calcareous alluvial deposits, 

 are the remains of a series of beds, which once constituted 

 an ancient plain, at a higher elevation than the present. 

 Many are surmounted by a crest or capping of basalt, to 

 which their preservation is probably attributable ; others 

 have escaped destruction from being protected by hori- 

 zontal layers of a durable limestone, which I shall pre- 

 sently describe. 



We have, then, as the ground-plan of the district, an 

 extensive plain, chequered with low hills of fresh-water 

 limestone, which are capped with compact lava {PL II. 

 Jig. in) ; the boundaries of this tract being formed of 

 ranges of primary rocks, 3,000 feet in altitude. To the 

 westward the limestone disappears, and a plateau of granite 

 rises to a height of about 1,600 feet above the valley of 

 Clermont, being 3,000 feet above the level of the sea. 

 This supports a chain of volcanic cones and dome-shaped 



