﻿"Vol. 64.] QUANTITATIVE METHODS TO THE STTTDY OF EOCKS. 195 



difficult to verify this by collecting mud from deep water and 

 studying its microscopical structure ; at present, therefore, it is 

 necessary to make inferences from experiments. At the same time, 

 the subject is so complex that unknown conditions may vitiate 

 some of the conclusions. 



So far as I can judge, a gentle current of varying velocity in 

 muddy water would explain the structure of many rocks which 

 have alternating layers of diflferent character, some very fine-grained 

 and others more sandy, the thickness of merely an inch having a 

 complex history of deposition and microscopical denudation. How- 

 ever, at all events in the older rocks, cases are fairly common 

 which seem to require much more rapid slight alterations in the 

 current than seem likely to have affected the general mass of water. 

 Since the bottom of a current is retarded by friction and moves 

 more slowly than that higher up, it seems probable that eddies are 

 formed causing the water at the bottom to move with sufficiently- 

 varying velocity to modify the deposition, and to give rise to very 

 thin laminae. I am sorry that I have not been able to verify this 

 experimentally ; but the structure of the ripple-drift in the green 

 slates of Langdale, described later (PI. XY), seems to prove the 

 existence of such pulsations, whatever may be the true cause. In 

 the ripple-drift on the side where the material was thrown down 

 on a slope, in what represents the ' ripple-period,' may be counted 

 eighty layers of slightly-varying material ; and, since the most 

 probable length of this period is half a minute, they would indicate 

 about 160 pulsations in a minute. Similar thin layers are seen in 

 a part where the current was not sufficiently strong to produce 

 ripples, but they are thicker, as though the pulsations were slower. 

 Since the calculated period depends on so many factors, all subject 

 to error, it would perhaps be best to assume that when the current 

 is too slow to produce ripples or drift-sand, the pulsations were, 

 roughly speaking, of about the magnitude of one per second, which 

 would indicate that the thickness of a single layer was deposited 

 in that time. This must be looked upon as only an approximation ; 

 but, a.t all events, it is an interesting and plausible conclusion, and 

 agrees fairly well with the probable period of the pulsations which 

 must accompany the bottom-ripples. 



Application to Particular Rocks, 



Although I possess a fairly-large collection of thin sections of 

 slate-rocks, yet the variations due to chemical and mechanical 

 changes are so numerous that I have but few throwing light 

 on this particular question. I have, however, some excellent 

 examples. In one from the Skiddaw Slate of Portinscale there are 

 darker and paler layers of different-sized grain, evidently due to 

 water depositing different material ; but these layers are divided 

 into more or less well-marked laminae, varying from -002 to -010 

 inch in thickness, and therefore, perhaps, indicating that the rate of 



