8o Physical 



bottom of the sea to its summit. In a case such as this, 

 it is easy to see that the ordinary marine sediments of 

 the area would get intermingled with volcanic ashes, 

 and possibly with submarine streams of lava. 



Explosions of steam accompanied by floating cinders 

 are mentioned by Darwin as occurring at intervals in 

 the South Atlantic ; and anyone who will tax his 

 memory a little will recollect that a large proportion of 

 the volcanoes of the world are islands, or in islands, in 

 the Atlantic, the Indian Ocean, the Indian Archipelago, 

 and the Pacific Ocean, south and north. It has been 

 often remarked that almost all volcanoes are in the 

 neighbourhood of the sea. 



I think, then, at the time of the deposition of the 

 Llandeilo and Bala beds of our area, our terrestrial 

 scenery consisted of groups of volcanic islands scattered 

 over the area of what is now North Wales and South 

 Wales, and extending westward into the region of the Irish 

 strata of the same age, and northward as far as the sea 

 that then rolled where Cumberland now stands ; for there 

 also volcanic rocks occur in great force, all of the same 

 general character as those found in Wales. There is 

 however, this difference between the two areas, that, 

 whereas in Wales ordinary sediments are plentifully in- 

 terstratified with lavas and ashes, and sometimes even 

 lithologically intermingled with volcanic ashes, in the 

 Cumbrian area it is only for a few feet at the very base of 

 the volcanic series that interstratifications take place, 

 the whole of the rest of these Silurian volcanic rocks 

 of Westmoreland and Cumberland being quite destitute 

 of any intermixture of marine sediments. Exclusive 

 of intrusive rocks, the whole consists of purely terres- 

 trial lavas, volcanic conglomerates and ashes, the latter 

 often well stratified, for where showers of ashes fall 



