VALLEYS 143 



expanse of black basalt stretching up to the base of 

 the mountains. What may have been the total thick- 

 ness of basalt cannot be told ; but the fragment remain- 

 ing in Ben More, Mull, is more than 3,000 feet thick. 

 So vast has been the erosion since older Tertiary time 

 that the volcanic plateau has been trenched in every 

 direction by deep glens and arms of the sea, and has 

 been reduced to detached islands. It is strange to 

 reflect that all this revolution in the topography has 

 been effected since the soft clays and sands of the 

 London Basin were deposited. 



THE VALLEYS. 



The intimate relation of a system of valleys to a 

 system ot drainage lines, first clearly enunciated by 

 Hutton and Playfair, has received ample illustrations 

 from all parts of the world. 1 But the notion is not yet 

 extinct that, in some way or other, valleys have been as 

 much, if not more, determined by subterranean lines of 

 dislocation than by superficial erosion. Some favourite 

 dogmas die hard, and though this dogma of fracture 

 has been demolished over and over again, it every now 

 and then reappears, dressed up anew as a fresh contri- 

 bution to scientific progress. We have only to compare 

 the surface of a much dislocated region with its under- 

 ground structure, where that has been revealed by 

 mining operations, as in our coal fields, to see that 

 valleys comparatively seldom, and then only as it were 

 by accident, run along lines of dislocation, but that 

 they everywhere cut across them, and that faults 

 rarely make a feature at the surface, except in- 



1 See pp. 161, 193. 



