﻿Vol. 64.] QUAl^TITATlVE METHODS TO THE STUDY OF EOCKS. 213 



included water had been squeezed out by the pressure of superin- 

 cumbent material : and their fissility is like that produced in my 

 experiments, being due, not to any alternation of different materials 

 in the plane of bedding, but to the necessary change in ultimate struc- 

 ture brought about by the great decrease in thickness. As shown 

 by these experiments, the interspaces in clay which had settled 

 so as to become somewhat stable amounted in extreme cases to no 

 less than 86 per cent, of the volume, whereas in the fine-grained 

 shales of the Coal-Measures they amount only to 13 per cent., which 

 indicates a probable reduction to about a sixth of the original 

 volume. Considering the nature of the material, this would be 

 quite adequate to develop an imperfect cleavage in the plane of 

 bedding, in accordance with the principles which I have described 

 in my papers on slaty cleavage. The experiments also explain the 

 change consequent on the weathering of the shale, which has 

 become soft, and now contains 39-2 per cent, of interspaces ; this 

 corresponds to what happens when the dried clay swells up by water 

 forcing itself into it, such an effect being no doubt increased in 

 the shale by frost. 



As bearing on the production of schistosity in shales and of 

 cleavage in slates, I may mention an experiment that I made with 

 what was given to me as ' Brodie's graphite.' This, when filled into 

 a brass tube about 2 inches long, was loose and bulky, but could be 

 compressed down to about a quarter of an inch by a tightly-fitting 

 brass rod, when it was found to be fairly solid, and to possess a 

 more perfect cleavage than any slate, in the plane perpendicular to 

 the direction of compression. 



Chalk. — My experiments with fine-grained chalk show that, 

 when deposited so as to arrive at a kind of temporary stability, it 

 is a sort of imperfect liquid ; and this was probably the case with 

 the natural deposit. The amount of interspaces might well have 

 been more than 70 per cent. My impression is that, many years 

 ago, I examined a soft chalk in which they were still about 50 

 per cent. Artificially deposited, very fine-grained chalk, when 

 contracted by drying without any pressure, contains 41 per cent, 

 of interspaces, or sensibly the same as shaken shot or sand. The 

 clunch used for internal artistic work was found, by the boiling- 

 water method, still to contain 34 per cent. The Chalk of the 

 Yorkshire coast has been much hardened by infiltrated calcite, 

 and contains only about 15 per cent. Probably, then, the reduction 

 in thickness of natural chalk has been to about 45 per cent, of 

 the original thickness, which is a much smaller reduction than 

 in shales ; and this, combined with the difi'erence in the character 

 of the material, will explain why, in the case of the natural 

 rocks, and in my experiments, no fissility was developed approaching 

 that of shales. 



Cavities in slate- rocks. — The determination of the exact 

 amount of cavities in rocks possessing cleavage is rendered difficult 



