70 The Founders of Geology LECT. 



formation is wrought on the face of the country. Lavas 

 which originally covered the floors of valleys, as the 

 ground around them is lowered, are at last turned into 

 high tablelands, and are still further cut through and 

 separated into detached portions, according to the multipli- 

 cation and deepening of the ravines and valleys by which 

 they are traversed. To realize the ancient continuity of 

 these venerable lava-sheets, we must in imagination fill up 

 the valleys, and thus restore the plain over which the 

 molten rock originally flowed. 



As all the scoriae and craters are gone, the only way of 

 detecting an eruptive centre in the volcanic products of 

 this epoch is to find the point of common origin for several 

 streams, such points being often marked by large isolated 

 patches of lava (culots). 



Desmarest arrives at the important conclusion that the 

 lavas of his second epoch were erupted before the excava- 

 tion of the present valleys out of the original plain over 

 which the streams of basalt were poured. The volcanic 

 events of which they are the memorials must thus go back 

 to a remote antiquity, for the erosion of valleys is obviously 

 an exceedingly slow process. But these lavas are evidently 

 much younger than the horizontal sedimentary strata and 

 the granite which they overlie, both of which are also 

 trenched by the valleys. 



The third and most ancient epoch is denoted by a series 

 of lavas, which, instead of overlying the sedimentary strata, 

 underlie them or are interstratified with them. These 

 sediments are now recognized as the deposits of one of the 

 old Tertiary lakes of Europe. Their layers are full of land- 

 plants, land and fresh-water shells, and remains of terres- 



