﻿Vol. 64.] QTJAIfTITATIYE METHODS TO THE STUDY OF EOCKS. 211 



recently-deposited sand of fine grain. It seems to me probable that 

 slight deposition of quartz may play an important part in causing 

 the grains to cohere, in many sandstones which are not disintegrated 

 by long exposure to strong acids, and thus resist exposure to the 

 "weather remarkably long. 



Limestones of Oolitic age. — I have made many determina- 

 tions of the open interspaces by thq hot-water method ; but their 

 present structure is due to so many complex conditions, that the 

 results are of little value in connexion with my present purpose. 

 In the case of building-stones, the maximum was 37 per cent, and 

 the mean of the maximum values about 31. The minimum for the 

 more solid was 13 per cent, and the average about 15. This is 

 as though such limestones were more or less solidified by infiltrated 

 calcite not long after deposition, and as though the process had 

 not been carried out so completely as in the case of many older 

 rocks. 



Kentish Eag. — By the boiling-water method I found that, in 

 its natural condition, this rock contains only about 3*5 per cent. 

 of open interspaces ; whereas the much-weathered rock contains 

 26*1 per cent., owing to the removal of soluble material, which left 

 the rocks with about the same amount of interspaces as in the case 

 of the most closely-packed fragments. 



The cavities in many other rocks will be considered, when 

 dealing with the pressure to which they have been subjected. 



Determination of the Cavities in Fine-Grained Rocks. 



The physical characters of different fine-grained rocks vary so 

 much that the fairly-accurate determination of the cavities is in 

 some cases quite easy, but in others difficult. This is due to the 

 combination of a number of curious relations between a certain 

 class of rocks and water, which they seem to contain in four or five 

 different conditions : — (1) The water is combined chemically, but 

 lost on strong heating ; (2) it is absorbed, somewhat like an 

 occluded gas : (3) it is condensed from a damp atmosphere among 

 the very minute particles, but not in such quantity as to fill the 

 cavities ; (4) when the rock is placed in water, the latter fills up 

 the cavities ; and (5) it may not only fill them, but interpenetrate 

 the material with such force as to swell it to a considerable extent, 

 or even break it up completely and cause it to occupy about twice 

 and a third the volume of the solid material. One result of the 

 absorption of water from a damp atmosphere is that it may be 

 almost impossible to decide what ought to be looked upon as the 

 correct weight. This is certainly not due to mere fine division, 

 since very finely-powdered calcite does not increase in weight 

 in an atmosphere saturated with water, whereas clays largely 

 composed of decomposed felspar vary considerably in weight from 

 day to day, according to the state of the weather, the amount 



