﻿Yol. 64.] QUANTITATIVE METHODS TO THE STUDY OF ROCKS. 199 



Ripple-Drift in other Rocks. 



The detailed study of ripple-drift in other rocks is far more 

 difficult than in the Langdale Slates, because the colour of the 

 material does not varj- so much, and my collected specimens do not 

 enable me to give such precise results. One, however, which is 

 fairly characteristic of Coal-Measure sandstones, indicates a current 

 of about 1 foot per second, and dei)osition at the rate of about 

 two-thirds of an inch per minute, as the rock now is, or 1 inch per 

 minute in its original condition. Beds of varying ripple-drift of 

 considerable thickness are common, and they differ widely from the 

 Langdale Slates just described. The careful study of the rocks 

 in situ would probably yield interesting results, and furnish infor- 

 mation respecting the exact nature of the currents. There certainly 

 appears good evidence to show that deposition up to 1| inches 

 per minute was common, but associated with possibly long intervals 

 during which there was little deposition. It must also be borne 

 in mind that there may have been currents of over 18 inches per 

 second, which would wash up the sand and leave little or no 

 evidence of their existence, if no coarse material existed in the 

 district. 



I possess a specimen from the Lower Coal-Measures at Ringingiow 

 near Sheffield, which, like the Langdale Slates, shows a current of 

 about 1 foot per second, acting for a short time, so as to produce 

 ripples, and not merely graining of the surface (like in the rest of 

 the rock), as if due to currents varying frequently up to somewhat 

 less than 6 inches per second. 



XI. Washing-up, etc. op Clays. 



The velocity of current needed to just wash up very fine-grained 

 deposits must necessarily depend so much on the length of time 

 that has elapsed since they were deposited and their state of con- 

 solidation, as well as on the effect of the associated small animals 

 and plants, that in many cases all calculation is impossible. 

 Judging from my experiments, recently-deposited fine clay, un- 

 modified by minute organisms, would be washed up by a very gentle 

 current, since its density differs so little from that of water ; 

 and we may safely conclude that it would not be permanently 

 accumulated, except in more or less still water. On the contrary, 

 I have dredged up fine-grained mud, made so tenacious by Jassa 

 jaulchella and other small organisms, that it was almost impossible 

 to wash it out of the dredge. Banks of such mud would resist a 

 much stronger current than one that would wash away sand, so 

 that calculation from the size of the grains is out of the question. 

 However, it is more important to consider the velocity of current 

 that would allow of deposition ; but, even then, very much would 

 depend on the quantity held in suspension and on the extent to 

 which the particles collected into compound groups. Assuming 

 that no deposit would be formed when the current was stronger 



