﻿212 DE. H. C. SORBT ON THE APPLICATION OF [MaV I908, 



varying up to at least 5^ per cent, Very much remains to be 

 learned respecting the connexion of these facts with the history of 

 the rocks ; hut they are evidently of fundamental importance, and 

 in some way are connected with the age of the rocks. One striking 

 fact is, that specimens of diiferent geological periods may look almost 

 alike, and yet differ enormously in their reaction with water. 



Then, again, we have to consider the amount of soluble salts 

 dissolved out by water, as shown by the loss of weight. The amount 

 and the nature of these form a subject worthy of careful study. 

 My results are but imperfect, because deduced from experiments 

 made specially to learn other facts. So far as they go, they make 

 it appear possible that in fine-grained shales there are to some 

 extent the salts originally contained in the water from which the 

 shales were deposited; this was slowly lost by evaporation, since 

 the amount of salts is sometimes far greater than in the volume 

 of sea-water that would fill the cavities. There is also in the 

 specimens examined more salt in the shales deposited in sea-water 

 than in corresponding rocks from the freshwater Coal-Measures. 



When fine-grained rocks are treated with water, there is generally 

 a certain amount of swelling, which may be so small as to be invisible, 

 and there is no breaking-up ; but sometimes the tumescence is so 

 great and energetic, that the specimen quickly breaks up from a fairly- 

 hard rock into a soft clay. In such cases, it is of course impossible 

 to determine the amount of natural cavities by means of water, and 

 almond-oil or benzol must be used. In this event allowance must 

 be made for what may be dissolved by these liquids. When the rock 

 does not break up, the volume of absorbed water may be considerable, 

 and due almost entirely to swelling, the amount of oil or benzol 

 absorbed being comparatively small. After being soaked with water, 

 the specimen may, or may not, contract to its original volume, on 

 drj'ing. Then, again, after the specimen has been kept in water 

 for several days, its weight when dried may be considerably less 

 than it was, owing to the removal of soluble salts, which may 

 amount to several per cent. 



The Structure of Fine-Grained Deposits. 



The results of my experiments with clay and chalk explain the 

 structure of many deposits of nearly all ages. It appears as though 

 the comparatively-recent mud of our estuaries may remain for a 

 very long time in a sort of semi-fluid condition. That in the wide 

 mud-flat in the Deben, opposite Waldringfield, which may have been 

 there for ages, is so soft that an oar can be pushed down into it for 

 a good many feet with scarcely any resistance, the mud being in 

 much the same condition as the clay in my experiments, which did 

 not subside more on keeping. In other cases, the upper part is very 

 soft ; but the material becomes firmer at a lower level, where it may 

 have existed for ages under some little pressure. The much older 

 clays and shales agree with what would happen if more and more 



