434 Dr. H. Sicks — British Pre-Cambrian Rocks. 



undergoing periodical contractions along certain defined lines. That 

 these lines have also periodically varied in their direction is well 

 known. If we require therefore to trace out the history of these 

 movements, we must seek for it in those strata which we know 

 must have been elevated at the time ; and whose beds must have 

 generally assumed a strike parallel with the upward movements. 

 Sir E. Murchison years ago pointed out the fact that the majority 

 of the crystalline rocks in the Hebrides and North-western High- 

 lands of Scotland had a strike from N.W. to S.E. ; and hence 

 in a direction not usual in the Cambrian and Silurian rocks. 



This fact has been more or less qualified by all subsequent 

 observations, and it is one which cannot be lost sight of in any 

 inquiry made as to the early physical history of our islands. My 

 own observations tend to show that the oldest known upward 

 movements were in a direction almost due east and west, that is, 

 that the contractions took place from north to south ; subsequent 

 elevations and depressions in a direction from N.W. to S.E. ; more 

 recent ones from N. to S., and those which immediately preceded 

 the Cambrian period from N.E. to S.W. Each of these produced 

 new physical conditions, with breaks in the succession along well- 

 defined, and usually extensive, lines. Another important fact made 

 out in these inquiries is that it may be possible sometimes to 

 determine the age of the faults by the direction which they have 

 taken, especially if of great extent. For example, a tolerably rigid 

 crust under the influence of a lateral thrust acting from E. to W. 

 would most probably fracture in a direction from N. to S. Now if 

 we take for granted that the last sediments deposited previous to 

 this lateral pressure being exerted were in a condition to yield 

 readily to the contractions, as they probably would be ; but that 

 the underlying rocks would be in a hardened or metamorphosed 

 state ; it is clear that these last only would be fractured. If these 

 movements also did take place in the direction of those which had 

 previously acted upon these rocks, then generally the fractures 

 would be across the beds, and in the direction of the curvatures of 

 the overlying sediments. That is' to say, the faults affecting A 

 would be in the direction of the strike in B. 



At present it is difficult to separate always those which may be 

 called the primary faults from those produced subsequently during 

 fresh contractions ; still they may be recognized frequently, and when 

 they are they aid greatly any attempt made to unravel the early 

 physical changes. The oldest faults which can be traced with cer- 

 tainty are in a direction from N. to S. and were produced at the close 

 of the Arvonian. They cut across the beds of the Lewisian, and 

 Dimetian, and exposed these rocks as ridges in the Pebidian sea, and 

 Pebidian beds are now seen resting along the sides and unconform- 

 ably across the edges of these rocks. These faults therefore prove, 

 along with other evidence, that the former rocks were crystalline 

 ere the latter were deposited, and that they yielded by fracturing to 

 the lateral pressure exerted from E. to W. which gave the direction 

 of strike to the Arvonian group. 



