﻿Vol. 64.] QUANTITATIYE METHODS APPLIED TO STUDY OF EOCKS. 171 



10. On the Application of Quantitative Methods to the Study 

 of the Structure and History of Rocks. By the late 

 Henry Clifton Sorby, LL.D., F.E.S., F.L.S., F.G.S. (Head 

 January 8th, 1908.) 



[Plates XIV-XVIII.] 



Contents, 



Page 



I. Introduction 171 



II. Final Velocities 172 



III. Angles of Eest of Sand and of Small Pebbles 174 



IV. The Effects of Currents 176 



V. Ripple-Drift 181 



VI. Varving Size of the Grains 185 



VIL Drift-Bedding 186 



VIII. Joints of Encrinites, etc 189 



IX. Very Fine-Grained Deposits 189 



X. The Green Slates of Langdale 196 



XI. Washing-up, etc. of Clays iy9 



XII. On the Interspaces between the Constituent Grains of 



Deposited Material 200 



XIII. Segregation 203 



XIV. Contraction of Eocks after Deposition 214 



XV. Concretions 215 



XVL Spots in Welsh Slates 220 



XVII. Slip-Surfaces 222 



XVIII. Surfaces of Pressure- Solution 224 



XIX. Determination of the Pressure to which Rocks have 



been Subjected ... 227 



I. Introduction. 



In the case of nearly all branches of science a great advance was 

 made when accurate quantitative methods were used instead of 

 merely qualitative. One great advantage of this is that it 

 necessitates more accurate thought, points out what remains to be 

 learned, and sometimes small residual quantities, which otherwise 

 would escape attention, indicate important facts. Since it applies 

 to nearly all branches of geology, it is necessarily a wide subject, 

 but so connected together that it seems undesirable to divide it. 



My object is to apply experimental physics to the study of 

 rocks. 



At least six different kinds of physical questions are involved, 

 some of which have been sufficiently studied, but others require 

 experiments which would be very difficult to carry out, and all that 

 I can now do is to endeavour to deduce plausible results from what 

 is known. In doing this, it may be necessary to assume cases 

 sufficiently simple for calculation, which may but imperfectly 

 correspond to natural conditions, so that the results may be only 

 approximately correct. In some cases, facts seem to show that 

 there are important properties connected with subsi^ng material 



