3G0 



parallel to such planes. The planes of stratification, on the other 

 hand, are perfectly distinct from both, and throughout the district 

 alluded to have never been found to coincide with the lines of cleav- 

 age, dipping sometimes to the same point and sometimes to opposite 

 points of the compass, but being always inclined to them at an angle 

 of from 10° to 30° or 40°, and in no instance at 90°. There are re- 

 gions in North and South Wales thirty miles in extent, and many 

 miles in breadth, where the cleavage planes preserve an undeviating 

 dip and direction notwithstanding that they traverse strata which 

 are greatly contorted. 



In that variety of slate-rock which is used for roofing, all traces 

 of original deposition or stratification are often obliterated ; yet in 

 many quarries, a number of parallel stripes are discovered, sometimes 

 of a lighter and sometimes of a darker colour than the general mass. 

 These stripes, says the Professor, are universally parallel to the 

 true beds, whenever such beds can be discovered, whether by or- 

 ganic remains, by the alternations of similar deposits, or other 

 ordinary means. Many of these beds are of a coarse mechanical 

 structure, others are fine chloritic slate ; but the coarser beds and 

 the finer, the twisted and the straight, have all been subjected to 

 one change, a crystalline cleavage passing alike through all. Some 

 of the sections given show the cleavage planes preserving an almost 

 geometrical parallelism while they pass through curved strata, of 

 which the sedimentary origin is obvious. In another place it 

 is said that where the slaty cleavage is very perfectly brought 

 out the rocks always make an approach to homogeneity, but where 

 the coarse beds predominate the slaty structure almost entirely dis- 

 appears. Dr. Boase in his comments on these passages has re- 

 marked that they seem inconsistent with each other, and I confess 

 that at first they struck me in the same light ; but the Professor has 

 explained to me that although the coarse beds are not slaty, they 

 have a grain parallel to the cleavage planes of the finer beds, this 

 grain being exhibited when they are struck with the hammer ; and it 

 is only when the materials of the beds are very coarse that the 

 cleavage planes entirely vanish. 



In regard to the origin of these phaenomena, the author supposes 

 that crystalline or polar forces must have acted on the whole mass 

 simultaneously in given directions, and that the action being carried 

 on at once through a very large mass of matter may have acquired 

 an accumulated intensity of crystaUine action in each part, so that 

 the whole intensity of crystalline force, modifying the mass, may 

 not have been equal to the sum of the forces necessary to cry- 

 stallize each part independently, but may have been some function 

 of that sum whereby it may have been increased almost indefinitely. 



I regret that I have not space to do justice to this ingenious 

 speculation, nor have I yet had sufficient opportunities of obser- 

 vation to know whether w'e shall be able to distinguish generally, 

 with precision, those slates which are diagonal to the strata, from 

 those flagstone-slates, as it is proposed to term them, which are pa- 

 rallel to the layers of deposition. During the last summer I observed 



