﻿194 DE. H. C. SORBT ON THE APPLICATION OP [May I908, 



To investigate fully what occurs under natural conditions would 

 be a difficult undertaking, because some of the most important 

 facts could be learned only in rough weather when it would be 

 impossible to collect material. The result is that some of the 

 most striking peculiarities or many of our rocks cannot be satis- 

 factorily explained experimentally, or by appeal to what uow takes 

 place. 



When dry hydrous pipe-clay was pressed together by hand it 

 contained 43*4 per cent, of interspaces. On applying more pres- 

 sure they amounted to 24*4 per cent., or closely as in the case of 

 spheres arranged to occupy the least volume ; but on further pressure 

 they were reduced to 21*4. These results were very different in 

 the case of ignited pipe-clay, for even after great pressure the inter- 

 spaces were 48 per cent. Hence it seems clear that when the clay 

 is hydrous, the grains are sufficiently soft and mobile to yield and 

 partly fill up the interspaces, which is an important fact to bear 

 in mind in studying the structure of many rocks. 



The change produced by varying amounts of water in clay is of 

 much interest. When only 40 per cent, of the volume of water is 

 present, hydrous pipe-clay breaks easily and is not plastic. With 

 45 per cent, it is just plastic and soft, and completely plastic with 

 50 per cent. It thus appears that complete plasticity depends on 

 the presence of rather more water than is sufficient to fill the 

 interspaces. When less than this, its action is probably that of 

 suction, but when more, it acts as a lubricant. This must also be 

 important in connexion with the consolidation of many natural 

 rocks. 



The Effects of Currents. 



Although the laws of the deposition of clays in still water had 

 first to be learned, yet it seems quite certain that in many cases 

 the results must have been greatly modified by gentle currents. 

 Suitable experiments would require a considerable stream of clear 

 water and special arrangements. However, judging from the rate 

 of subsidence, the velocity of current necessary to produce decided 

 effects must vary much according to the nature of the mud, and 

 must be of some such order of magnitude as from 1 inch to 1 foot 

 per hour for very fine mud, and 1 foot per minute for coarser. 



It is easy to understand how irregularities in velocity can be 

 produced. I have paid a good deal of attention to the movements 

 of currents in tidal estuaries. The velocity varies much vertically, 

 and although there is often a strong current in mid-stream, there 

 is none (or else an eddy) towards the shore. Moreover, vortices 

 are often formed with horizontal axes more or less perpendicular to 

 the direction of the current, which on one side may bring up mud 

 and carry it down the other. The result of all such actions would 

 be to produce more or less variations in the suspended mud ; and, 

 when the movement of the water becomes sufficiently slow, layers 

 of different texture may be deposited. It would be extremely 



