MONT DORE. 



271 



This region affords, too, a striking illustration of the 

 erosion of the surface of a country by alluvial action. The 

 thickness of the volcanic mass is between 300 and 400 feet ; 

 it is composed of two distinct beds of basalt, separated by , 

 a layer of scorise and volcanic fragments. Many portions, 

 both of the upper and lower beds, are made up of well- 

 defined, vertical, polygonal columns. The streams of lava 

 to which these plateaux belong, have been traced for more 

 than thirty miles ; they rise in a narrow ridge across the 

 primitive heights, and then spread over, and lie com- 

 formably upon, the secondary formations. The limestone 

 beneath the basalt is, in some places, covered with vege- 

 table soil, containing a common species of terrestrial shell 

 (Cyclostoma elegans). The nearly horizontal disposition of 

 the basalt, its columnar structure, and position on the lime- 

 stone, into which it has injected veins and dikes, render it, 

 as Mr. Scrope observes, very analogous to the ancient 

 volcanic rocks of Ireland, which will be described in a 

 future Lecture. 



49. Mont Dore {PI. II. Jig, i). In the same province 

 there is another remarkable system of extinct volcanoes con- 

 nected with the Puy de Dome. While in the district I have 

 just described, the primitive soil is only partially obscured by 

 the volcanic products, in Mont Dore, the granitic foundation, 

 over an area of many miles in extent, is entirely covered by 

 them, and the erupted masses attain a considerable elevation. 

 Mont Dore is a mountainous tract, the highest portion of 

 which is about 6,000 feet in altitude.* It consists of a group 

 of seven or eight rocky summits, which form a zone a mile in 

 diameter, the whole consisting of a succession of beds of 

 volcanic origin. It is deeply channelled by two principal 



* PI. II. fig. i. is a profile of Mont Dore as seen from a distance, 

 from a sketch by M. Constant Prevost : the dotted outline shows the 

 form of the cone when in activity ; a ground plan of the broken crater 

 is annexed. 



