﻿214 DE. H. C. SOEBT ON THE APPLICATIOX OF [May I908, 



by a peculiarity which, in most cases, is of no importance. After 

 boiliDg in water, there can be little doubt that, as a rule, nearly all 

 the loss of weight on drying is due to liquid water which existed in 

 the cavities. In the case of slate-rocks, which are so nearly solid, 

 what otherwise would be small effects become of importance. 

 Thus, in the case of the Penrhyn slate, after being boiled in water, 

 and kept in the water until the weight was constant, the speci- 

 men on being dried in the atmosphere at the natural temperature 

 indicated (by loss) the existence of about -24 per cent, of interspaces; 

 but, when it was dried at a very moderate heat near a fire, there was 

 a further small loss, which, if due to liquid water which had been 

 in cavities, would indicate that these amounted to nearly a half per 

 cent. This should make a very considerable difference in the calcu- 

 lated pressure, discussed later (p. 227). It seems, however, impro- 

 bable that liquid water would remain in the rocks when dried for 

 several days in the air of a warm sitting-room, and more probable 

 that the further loss in weight represents water occluded iu, or 

 loosely combined in some way with, thie solid material. If so, the 

 question arises whether part or even all of the water lost after 

 boiling is not also thus combined, and does not represent cavities. 

 On the whole, in the present state of the question, it is probably 

 the best to adopt a mean result, and to conclude that the empty 

 spaces in the Penrhyn slate amount to '24 per cent. 



XIY. COXTEACTION or EOCKS AFTEE DEPOSITION. 



This may be very little, or as much as 90 per cent, of the original 

 thickness, even when nothing has been removed chemically. In 

 the case of sandstones and allied rocks, having at first about 46 per 

 cent, of cavities, the contraction when the grains had accommodated 

 themselves to the least volume would be about 25 per cent., but 

 much less if the sand consisted of grains of extremely-variable 

 size. Much depends, however, on whether the cavities had or 

 had not been filled with calcite at an early period ; and a rock 

 may now be almost quite solid, and yet may have contracted very 

 little. Manifestly, then, we must rely on the evidence furnished 

 by each particular case. As shown already, the best is that 

 afforded by the change in the angle of rest of well-developed drift- 

 bedding or ripple-drift. Information of a less reliable character 

 may be furnished by concretions, as will be shown when dealing 

 with them. 



The contraction in the case of very fine-grained rocks may have 

 been very much greater, since, even after standing for a year without 

 further subsidence, such material may contain 90 per cent, of water; 

 whereas consolidated older rocks of analogous character may have 

 been made almost solid by the squeezing-out of this water — so 

 that, in extreme cases, shales and slates may occupy only a ninth 

 of the volume which they possessed when originally deposited. At 

 all events, it is important to bear in mind that the thickness of 

 many of our rocks may now be very much less than the original. 



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