"24 Messrs Lyell and Murchison on the 



been excavated out of the " cheire" of Come below Pontgibaud ; 

 3dly, All the basalt and subjacent gneiss destroyed as above 

 stated between Pranal and Combres ; and, 4thly, The whole 

 detritus of primary rocks brought down from countries higher 

 on the Sioule than the above localities. This enormous quan- 

 tity of rock and soil has been carried, we say, through this gorge, 

 in times comparatively recent ; for the products of Come and 

 Chaluzet are modern in comparison with the ancient basalts of 

 Auvergne, as are the latter in reference to the lacustrine tertiary 

 strata whereon they rest. 



If, then, within a period of time inconsiderable with reference 

 to the earth's history, the volume of matter carried down by a 

 single tributary stream of the AUier be so prodigious, it is easy 

 to perceive, that, were all the materials which have been trans- 

 ported during the same time by other rivers from the central 

 granitic mountains of France, united and presented to us in one 

 view, they would bear witness to the force of causes that have 

 operated, and are still operating, in nature, to an extent far be- 

 yond the calculation of many geologists. 



Volcano of Montpezat. 



Without regard to geographical connection, we shall next de- 

 scribe some phenomena analogous to those of Chaluzet, as pre- 

 sented by the valley of Montpezat in the Vivarais. For the 

 leading facts relating to this lava-current, we refer to Mr Scrope's 

 work*. He, in common with M. Faujas de St Fond, has de- 

 scribed the primary rock of this district as granite. But the 

 whole country, from the source of the Ardeche in the Haut 

 Vivarais to near Aubenas consists of gneiss, composed in gene- 

 ral of the ordinary ingredients of granite, but distinctly lami- 

 nated in strata, for the most part of great thickness, often verti- 

 cal, and always highly inclined, and here rarely deviating from 

 the same direction, viz. from NW. to SE. Some members of 

 this formation are composed here, as in Scotland, chiefly of horn- 

 blende; others are, in mineralogical characters, true granites, 

 though belonging to the same system, and granite veins are not 

 uncommon. The vertical! ty of the strata has contributed to 



" Geology of Central France. 



