Dr. Tyndall on Slaty Cleavage. 129 



commonly instrumental in giving rise to it than is generally sup- 

 posed. For instance, the slates of Penrhyn and Llanberis con- 

 tain about half their bulk of mica, which cannot indeed be seen with 

 the naked eye, because the flakes are on an average only yoVo *^ 

 of an inch long and ^-^^^^th thick, and but few are much larger. 

 "When, however, thin sections of these rocks are examined with 

 a microscope these flakes are most distinctly seen, especially in 

 the black slate of Llanberis village, which to the naked eye is 

 just like a fine-grained black shale, scarcely at all micaceous in 

 the usual application of that term. Chemical analysis also fully 

 bears out this conclusion ; but I may perhaps as well state here 

 that the characters of this mica appear to me to indicate that 

 it was not deposited in its present condition, but agree very 

 much better with the supposition of having been formed by a 

 peculiar transformation of a deposit of clay, of a different kind 

 to that which has occurred in mica-schist and rocks usually 

 called metamorphic, as I shall describe in a paper I hope to be 

 able to publish before long, on the chemical and physical struc- 

 ture of the older rocks. This size of the grains in these slates 

 is then very similar to that of the crystals of wax, or in other 

 words, the grain is as fine as that of wax. Their particles also 

 are of such an unequiaxed character as completely agrees with 

 their remarkably perfect cleavage, and I could only attribute 

 any other view of the subject to trusting to the natural appear- 

 ance of the rock, without examining a thin section with suitable 

 microscopical means. 



In conclusion I wish particularly to call attention to the fact, 

 that in forming a theory of the origin of the cleavage of slate- 

 rocks, we must not rest satisfied with one that merely accounts 

 for the splitting — it must be such as will explain their actual 

 ultimate constitution — the mountains must indeed be examined 

 with the microscope ; for it shows that the cleavage is not mere 

 joints or weak places, which do also occur most abundantly, but 

 chiefly depends upon a structure extending to the mechanical 

 arrangement of the ultimate grains of which they are composed. 



XIX. Observations on the preceding Paper. 

 By John Tyndall, F.R.S. 5fc. 



THE history of the paper which has called forth the foregoing 

 remarks is briefly as follows. In examining the influence 

 of structure upon the magnetism of bodies, I had occasion to 

 make numerous experiments on slate rocks, and in this way the 

 desire arose on my part to make myself acquainted with the 

 agencies by which the structure of such rocks was produced. 

 Phil Mag, S. 4. Vol. 12. No 71, Aug, 1856. K 



