64 The Founders of Geology LECT. 



floods stiffened into stone. Others have lost their cover- 

 ing of scoriae, and are seen clinging to the sides of valleys, 

 in positions which seem impossible for any lava-current 

 to have taken. Others are perched in solitary outlying 

 sheets on the tops of plateaux, with no cone near them, 

 nor any obvious source from which they could have flowed. 



Pondering on these apparently contradictory pheno- 

 mena, Desmarest, with the inspiration of true genius, 

 seized on the fruitful principle that would alone explain 

 them. He saw that the varying conditions of the several 

 lavas were due to the ceaseless influence of atmospheric 

 denudation. He convinced himself that the detached 

 outliers of basalt, capping the ridges and plateaux, are 

 really remnants of once continuous sheets of lava, and 

 that their isolation, together with the removal of their 

 original covering of scoriae and slags, is to be ascribed to 

 the operations of rain and melted snow. The depth of 

 the valleys cut through these lava-platforms was found by 

 him to be commensurate with the antiquity of the lavas, 

 and with the size of the streams that flowed between the 

 severed escarpments. 



He ascertained that, in proportion to their antiquity, the 

 lava-streams had lost, one after another, the usual outward 

 features of the younger sheets. The superficial scoriae had 

 disappeared, and the craters were worn away, until only 

 scattered outliers of compact dark rock remained. Yet be- 

 tween this extreme and that of the most recent eruptions, 

 where the lavas, in unbroken, rugged, cavernous sheets, 

 extend from their craters down into the present valleys, 

 where they have driven aside the running streams, every 

 intermediate stage could be found. 



