SHARPE ON NORTH WALES. 309 



formed in the depths of an ocean below the limit of animal life, or 

 as a formation which preceded the appearance of animals in these 

 parts of the northern hemisphere. 



Third Point of Difference.-^ln North Wales, not only in the 

 Cambrian but also in the Lower Silurian rocks, slaty cleavage is 

 universal : it is very common in the Wenlock series ; and in many 

 localities it runs, in a marked degree, through the whole thickness 

 of the Ludlow shales ; but in other localities these shales are wholly 

 exempt from it. The principal epoch of slaty cleavage, however, 

 preceded the formation of the Ludlow beds ; for these beds are in 

 many instances undisturbed by the faults which have broken up the 

 planes of cleavage in the older rocks. The author was assiduous in 

 measuring the position of the planes of bedding and cleavage in 

 various parts of North Wales, in the hope of making out some 

 general laws respecting cleavage. Li measuring the angles, how- 

 ever, he does not pretend to have approximated nearer than within 

 5° of the truth, as the surface of the beds is rarely flat enough to 

 allow of greater accuracy. 



One law respecting slaty cleavage was announced in 1831 by 

 Professor Sedgwick*, and is now well known: that law is, that 

 the cleavage planes maintain their parallelism over extensive areas, 

 irrespective of the varying position of the beds which they cut 

 through, or of the mineral character of the beds. Another law 

 respecting slaty cleavage was detected by the authorf in the progress 

 of his tour, and is the following : viz. that the strike of the cleavage 

 coincides with the strike of the bedding, whenever the latter con- 

 tinues uninterruptedly the same for a considerable distance ; but 

 when the strike of the beds is inconstant, and shifts at short inter- 

 vals, then the cleavage planes hold their course right on, irrespective 

 of the varying position of the planes of bedding ; in other words, 

 that the strike of the cleavage coincides with the prevailing strike 

 of the beds in each district, and does not vary with the subordinate 

 and local irregularities in the strike of the beds. Whence it follows, 

 that the strike of the cleavage in a district is far more constant and 

 regular than the strike of the beds. 



In order to present, in a succinct form, the evidence from which 

 the author has deduced this second law respecting slaty cleavage, 

 the observations he made, in various parts of North Wales, of tlic 

 positions of the planes of bedding and cleavage, are arranged in the 

 table given at p. 315. 



From the author's observations it appears, that the district of 



* Gcol. Trans. 2n{l ier. 3rd vol. p. 68. 



f While the author was drawing this conclusion from his ohservations in AValcs, 

 a nearly similar law was announced to the British Association at Cork hy Professor 

 JHiillips, in the following terms:— "The cleavage planes of the slate rocks of 

 North Wales arc always parallel to the main direction of the great anticlinal 

 axes, but are not affected by the small undulations or contortions of these lines. 

 In North Wales they maintain the same direction for fifty miles, not varying more 

 than two or three degrees." [See Athenajum, 2nd Sei)t. 1843.] 



