INTRODUCTION. 7 



breccia having a matrix of either quartzyte or vein- 

 quartz. As these rocks are harder, and have resisted 

 denudation better than the associated rocks, many 

 of the peaks are on a line of dyke, or at the crossing 

 of two ; while other dykes form the summit of ridges, 

 or stand out as reefs across the hills, or forming 

 shoulders to them. 



In other places no traces of the faults can be seen 

 on the surface of the ground. This has been espe- 

 cially remarked during the working of the different 

 coalfields, and it led an eminent geologist to state 

 that he could not see how there should be any con- 

 nection between faults and surface features, when 

 so many large faults in the coal measures are not 

 discernible on the surface of the ground. The oblitera- 

 tion of such fault-lines may be due to two causes : 

 these lines may be simple " slides," while the associ- 

 ated rocks are so similarly constituted that the de- 

 nudant (sea or ice) carved them evenly away ; while 

 in other cases the surface features due to faults may 

 have been obliterated by the subsequent deposition of 

 an envelope of drift. That drift often obscures these 

 features is proved by mining and other such work- 

 ings, as one pit may reach the rock in a few yards, 

 while another in its vicinity will have to be sunk 

 perhaps forty or fifty ; and in a drift country it is 

 generally found that most of the fault-lines, except 

 some that are post-glacial, are obliterated. It might 



