136 TOWN GEOLOGY. . [vi. 



beds over the ashes and lavas. Indeed, shells might 

 live and thrive in the ash-mud itself, when it cooled, 

 and the sea grew quiet, as they have lived and thriven 

 in Snowdonia. 



Now suppose that after these sedimentary beds 

 are laid down by water, the volcano breaks out again 

 what would happen ? 



Many things : specially this, which has often 

 happened already. 



The lava, kept down by the weight of these new- 

 rocks, searches for the point of least resistance, and 

 finds it in a more horizontal direction. It burrows 

 out through the softer ash-beds, and between the 

 sedimentary beds, spreading itself along horizontally. 

 This process accounts for the very puzzling, though 

 very common case in Snowdon and elsewhere, in 

 which we find lavas interstratified with rocks which 

 are plainly older than those lavas. Perhaps when 

 that is done the volcano has got rid of all its lava, and 

 is quiet. But if not, sooner or later, it bores up 

 through the new sedimentary rocks, faulting them by 

 earthquake shocks till it gets free vent, and begins its 

 layers of alternate ash and lava once more. 



And consider this fact also : If near the first (as 

 often happens) there is another volcano, the lava from 

 one may run over the lava from the other, and we 

 may have two lavas of different materials overlying 

 each other, which have come from different directions. 

 The ashes blown out of the two craters may mingle 

 also, and so, in the course of ages, the result may be 

 such a confusion of ashes, lavas, and sedimentary 

 rocks as we find throughout most mountain ranges in 

 Snowdon, in the Lake mountains, in the Auvergne in 



