ii Desmarest on Volcanic History 69 



be followed from the cones into the surrounding country 

 over which they have flowed. The most modern lava- 

 streams are not cut through by valleys, but form continuous 

 sheets. Yet within the limits of this first epoch proofs of 

 alteration manifest themselves. The loose scoriae and 

 cinders are washed down to lower levels, the cones are 

 attacked and the lavas begin to be trenched. As these 

 changes advance, the flow of running water gradually cuts 

 through the sheets of lava and forms valleys across them. 

 The epoch embraced all the ages required for this erosion, 

 and during its continuance repeated outflows of lava took 

 place. Each of these currents of melted rock would seek 

 the lowest levels, and would thus mark the valley-bottom 

 of its time, in the long process of excavation. 



In the records of the second epoch, the scorise and ashes 

 have been swept away, the cones have entirely disap- 

 peared, and the streams of lava have been cut into separate 

 patches by the erosion of the valleys, above which they are 

 now left perched as high plains or plateaux. Notwith- 

 standing the stupendous results thus achieved, Desmarest 

 seeks no vast terrestrial disturbance to account for them. 

 He finds their explanation in the working of the very 

 same meteoric agents which are still carrying on the same 

 process of degradation. The cellular parts of the lavas, 

 under the influence of the weather, crumble down into 

 mere loose earth, which is easily washed away by rain and 

 melted snow, leaving only the harder and more resisting 

 core of more solid rock. In like manner, the loose materials 

 of the cones are removed, until perhaps only masses 

 of lava remain behind that may have solidified at their 

 bottoms. By this series of operations an entire trans- 



