154 



OUTLINES OF FII 





Fig. 52 that cleavage runs independent of original bed- 

 ding, coinciding with it or not, as tin- strata may happen 

 to lie. The strike of the cleavage, which can be traced 

 with great persistence over large areas, as in North Wales, 

 marks the direction perpendicular to which the com- 

 pression of the rocks took place. In following it, there- 

 fore, the observer will keep a watch for every indi< 

 of other evidence as to the nature and extent of the 



Fie. 53. Deceptive appearance of unconformability due to variations in the 

 effects of cleavage upon different materials. West coast of Islay. 



terrestrial movements by which these great changes were 

 effected. 



In a cleaved region, the greatest risk of error which the 

 observer encounters is the liability to mistake cleavage 

 for bedding, and thus to form a false conception of the 

 geological structure of the ground. He ought to be con- 

 tinually on his guard against this source of deception, which 

 has misled even experienced geologists. Not only may 

 erroneous conclusions be drawn as to the true inclination 

 and thickness of cleaved rocks, but in some instances an 

 unconformability may be supposed to exist in what is 

 really a continuous succession of strata. This latter 

 mistake may readily be made where a mass of well- 



