,88 PROCEEDINGS pF THE GEOLOGICAL SOCIETY. [DeC. % 



.However, most important results may be drawn from attention to 

 "this subject. 



To begin with the strike of the cleavage planes : Professor Sedg- 

 Twick stated in 1835, as a rule subject to few exceptions, " where the 

 ^^cleavage is well-developed in a thick mass of slate rock, the strike 

 of the cleavage is nearly coincident with the strike of the beds*." I 

 adopted nearly the same view many years later, after examining 

 several extensive slate districts f. But a somewhat different state- 

 lament was made by Professor Phillips, that "the cleavage planes of 

 'the slate rocks of North Wales are always parallel to the main direc- 

 tion of the great anticlinal axes J " This view was taken by Mr. 

 ]^ Jukes, who appears to have paid much attention to the subject §. 

 "" Mr. Darwin has lately added his testimony to the same effect : in 

 "J summing up his observations upon cleavage in various parts of South 

 ^America, he says, "the cleavage laminae range over wide areas with 

 "remarkable uniformity, being parallel in strike to the main axes of 

 ^elevation, and generally to the outlines of the coast I|." 

 ^ The difference between these two views may appear slight, and in 

 t districts where, as is frequent, the strike of the beds agrees with 

 "the main direction of the great anticlinal axes," there is no dis- 

 cordance between them ; but the two statements really lead to widely 

 different theoretical conclusions. In a case like this, which turns on 

 generalizations drawn from observation, the greatest weight must be 

 given to the geologist whose opportunities of observation have been 

 the widest : in this all must yield to Mr. Darwin ; I have therefore 

 no hesitation in giving up my former opinion, and concluding that 

 the direction of the strike of the cleavage is parallel to the main 

 7 direction of the axes of elevation, and has no necessary connection 

 ' with the strike of the beds. 



The distance for which the cleavage strikes uniformly in nearly 

 the same direction is very remarkable, and has been often noticed : 

 over more than two-thirds of Wales the direction is between N.N.E. 

 and E.N.E., except where it has been modified by what must be 

 considered as local causes of disturbance. Extensive as this area 

 may appear, it is trifling compared with the regions in South 

 America over which Mr. Darwin found a uniformity in the strike 

 of the cleavage planes, and of what he regards as analogous to them, 

 the planes of foliation of the gneiss and mica schists. 



Mr. Darwin is the only geologist who appears to have sought for 

 order in the arrangement of the dip of the cleavage planes^ : he 



* Transactions of the Geological Society, 2nd Series, vol. ii. p. 473. 



t Quarterly Journal of the Geological Society, vol. ii. p. 309. 



t Report of British Association, 1843, p. 61. 



§ Excursions in Newfoundland, vol. ii. p. 324. 



II Geological Observations on South America, p. 163. The reader should study 

 the whole chapter, from p. 141. 



^ When the above passage was written, I had not seen the notice in the Edin- 

 burgh New Philosophical Journal, vol. xxxiii. p. 144, of a memoir on the " Clea- 

 vage of Slate Strata," presented by Professor H. D. Rogers at the sixth annual 

 meeting of American geologists and naturalists, April 1845, in which Professor 

 Rogers describes the direction of the cleavage planes through the Appalachian 



