Origin of Slaty Cleavage. 143 



form good slates, it will be seen that the arrangement of the particles 

 is very different. For instance, the well-known Water of Ayr stone 

 has no cleavage, but shews more or less of bedding. It consists of 

 mica and a very few grains of quartz sand, imbedded in a large pro- 

 portion of decomposed felspar ; the peroxide of iron being collected 

 to certain centres, and having the characters of peroxidised pyrites. 

 The flakes of mica do not lie in the plane of bedding, but are in- 

 clined tolerably evenly at all angles, so that there is no definite line 

 of structural weakness, independent of that due to bedding ; which 

 results chiefly from alternations of layers of somewhat different com- 

 position, and not from the arrangement of the ultimate particles. 

 This is however totally different in a rock of similar composition 

 having cleavage. If a section be examined, cut perpendicular to 

 cleavage, in the line of its dip, it will be seen that though some of 

 the minute flakes of mica lie perpendicular to the cleavage or at high 

 angles to it, by far the larger part are inclined at low, so that the 

 majority lie within 20° on each side of it. In fact they are most 

 numerous nearly in the plane of cleavage, and gradually but rapidly 

 diminish in quantity in passing to higher angles, so that there are 

 twenty times as many nearly in the plane of cleavage as at 45° to it, 

 and very few at 90°. Where a section is examined, cut perpen- 

 dicular to cleavage, in the line of the strike, it is seen that the ar- 

 rangement is similar, but there is not near so rapid a diminution of 

 the members in passing from the line of cleavage, so that there are 

 comparatively several times as many more inclined at about 45° to 

 it, than when the section is in the line of dip, and those at still 

 higher angles are also much more numerous. In a section in the 

 plane of cleavage, but few flakes are cut through so as to have a 

 greatly unequiaxed form ; but they are similarly arranged with re- 

 spect to the line of dip, though not in so marked a manner. It is 

 not merely the larger flakes of mica that are thus arranged, but 

 the whole of those unequiaxed particles which existed in the rock 

 before the cleavage was developed. 



When a cleavage crack in the thin sections is examined, it is 

 clearly seen that the cleavage is due to the above described arrange- 

 ment of the particles, which it follows most perfectly ; not passing 

 straight forwards, but turning about according to the manner in 

 which the ultimate particles lie in every part. It therefore appears 

 that the fissile character of slate is due to a line of structural weak- 

 ness, brought about by the manner of arrangement of the ultimate, 

 unequiaxed particles. The natural cleavage cracks, of course, bear 

 the same relation to this arrangement as those so often seen in 

 many crystalline bodies do to that of their ultimate atoms. They 

 appear, in general, to have been mainly due to meteoric agencies ; 

 their position having been determined by the structural weakness. 

 In accounting, then, for so-called slaty cleavage, it is only requisite 

 to shew how such particles could have had their position so changed 



