﻿Vol. 64.] QirANTITA.TIVE METHODS TO THE STUDY OF EOCKS. 231 



Table X. 



100— c. Tons per 



~c Feet of rock. sqicare inch. 



Little pressure 2-123 



Tertiary clays 2472 220 0402 



Boulder-Clay 3-032 573 0-265 



Liassic clays 3160 616 0-285 



Gault 3-167 658 0-304 



Coal-Measure shales 6407 2-700 1'250 



Penrhjn slate before cleavage... 8091 3-807 1"764 



Slate, Mawnan 15-949 8-715 4-040 



Moffat rocks 26-777 15550 7*200 



Hele, Ilfracombe 40-666 24-310 11-250 



Slates, Westmorland 203-080 126-800 68 700 



Slate, Penrhyu 416-700 261-500 121-000 



Compared with clays, shales, and slates, limestones and sand- 

 stones are unsatisfactory for the determination of pressure, since 

 those of different ages vary so much in important particulars. Still, 

 the mean results of a considerable number of specimens are never- 

 theless of some interest. Thus the average amount of interspaces 

 in fifty samples of Oolitic limestones is 32*2 per cent, of the solid 

 material. In the Magnesian Limestone it is 19-2, and in the 

 Carboniferous Limestone it is 11 per cent., whereas in the Carrara 

 marble it is only 0*7 per cent. These numbers agree fairly well 

 with the conclusions deduced from the clays. 



In the case of sandstones my results are, for the cavities in the 

 Kew Eed Sandstone 66 per cent, of the solid material ; for the 

 sandstones of the lower part of the Coal-Measures and the Millstone 

 Grit 25 per cent. ; and for the Old Eed Sandstone of Herefordshire 

 45 per cent. These results agree in a rough way with the others, 

 but the differences in the depth of the water and in the mineral 

 constitution make them valueless for calculating pressure. 



In conclusion, I may say that a number of my numerical values 

 must be looked upon as only approximate, though probably of 

 the true order of magnitude. More accurate results would often 

 require complicated experiments on a large scale, which could be 

 carried out only in a specially-organized laboratory, or in a small 

 clear river with artificial arrangements to control the current. 



EXPLANATION OF PLATES XIV-XVIII. 



[Plates XIV-XVI are slightly-reduced photographic reproductions from 

 the slates, and the other plates are reproduced from my own drawings.] 



Plate XIV. 



This shows the brenkiug-up of a fiae-grained deposit soon after deposition, 

 while still in a semi-hquid condition. (Green slate, Langdale.) See p. 197. 



