Geology 



These great thrust-planes, or overthrust faults, as 

 they are sometimes called, occur in many mountainous 

 regions, and are the ultimate result of the great thrusting 

 and crumpling movements of the Earth's crust to which 

 the mountains themselves are due. 



The mass of country above the thrust-plane had, in 

 some instances, been pushed forward over the rocks 

 below for a distance of several miles, and the mullion 

 structure was due to the tearing-up and shredding-out 

 of the rocks along the line of contact of the moving 

 masses. They were, in fact, slickenside on an enormous 

 scale. 



In the present instance it was impossible without a 

 very full and minute survey of the whole region to dis- 

 cover the amount of displacement due to the thrust, but 

 it was clear from the lie of the rocks that the movement 

 of the overlying mass was from south to north. 



Beneath the thrust-plane was a hard crystalline rock 

 which at first sight resembled granite, but a more careful 

 examination showed it to have a very different structure 

 to that rock, although consisting of much the same 

 minerals. 



Grains of quartz, felspar, and mica were present, but 

 they were arranged in thin layers, not flat like bedding- 

 planes in sedimentary rocks, but curved and undulating 

 as though they had been produced by the materials 

 flowing while in a treacly condition. 



This rock is called gneiss, and may have been pro- 

 duced from granite by intense crushing and shearing. 



There were also some darker masses possessing a 

 similar structure and containing large quantities of the 

 mineral hornblende, and these may have been produced 



212 



