Dr. Tyndall on Slaty Cleavage, 131 



of pyrites. From such slate there is every gradation to those con- 

 taining little or no mica, but made up of more or less fine quartz 

 sand and decomposed felspar in very variable proportions, hut these 

 have only an imperfect cleavage. Other slates, as is well known, 

 contain much chlorite and other minerals. On the present occasion 

 I shall chiefly confine myself to the consideration of such slate as 

 has a perfect cleavage. 



*' If a' thin section of a rock not having cleavage be examined, 

 which has a similar mineral composition to those which, when having 

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

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

 stone has no cleavage, but shows more or less of bedding. It con- 

 sists of mica and a very few grains of quartz sand imbedded in a large 

 proportion of decomposed felspar ; the peroxide of iron being col- 

 lected to certain centres, and having the characters of peroxidized 

 pyrites. The flakes of mica do not lie in the plane of bedding, but 

 are incHned 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 difl^er- 

 ent composition, and not from the arrangement of the ultimate par- 

 ticles. This is, however, totally diff^erent in a rock of similar com- 

 position having cleavage. If a section be examined, cut perpendi- 

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

 of the minutest 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 perpendi- 

 cular to cleavage in the line of the strike, it is seen that the arrange- 

 ment 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 un- 

 equiaxed form ; but they are similarly arranged with respect to the 

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

 IdiXgQv 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 weakness ^ 

 brought about by the manner of arrangement of the ultimate unequiaxed 

 particles. The natural cleavage cracks, of course, bear the same 



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