﻿Vol. 64.] QUANTITATIYE METHODS TO THE STTTDY OF ROCKS. 173 



supposing that au adherent film of water is dragged down with the 

 grains, the relative effect of which would be greater as their diameter 

 became less. The results are closely as if this film were about a 

 thousandth of an inch thick. My data were, however, not suffi- 

 cient to prove this, and yet they make it sufficiently probable to be 

 adopted provisionally. It may thus be supposed that the thickness 

 of the film may increase when the particles move more slowly 

 through the water ; and, if so, the final velocity must depend on very 

 complex conditions, and in the case of extremely-small particles it 

 may decrease more and more than the value of d. We may thus 

 easily understand why the subsidence of extremely-minute separate 

 particles is so very slow, even as slow as an inch per day. How- 

 ever, this is greatly influenced by the amount of suspended matter, 

 for, when there is much, the granules collect together into small 

 pellets, which subside far more rapidly than the separate smaller 

 granules. These facts must be carefully borne in mind in studying 

 very fine-grained rocks. 



It may be well to give in tabular form the approximate final 

 velocities in feet per second, deduced from many experiments for 

 separate grains of sand of various sizes, it being understood that the 

 results would vary to some extent with the shape of the grains. 



Table I. 



Shortest diameter .- inch '910 foot ^ „^„^„^ 

 L .con : sand. 



10 

 1^ 

 20 

 J_ 

 100 



J^ 



200 



1_ 



iooo 



•430 „ 1 



•106 „ / 



•052 „ "l 



•010 „ ; 



medium 

 sand. 



very fine 

 sand. 



These velocities are of fundamental importance in many questions 

 connected with stratified rocks. 



Flakes of Mica. 



Plakes of mica of small size and varying thickness follow the same 

 general laws as sand ; but, when the thickness remains constant, and 

 the area is greater, the final velocity increases with the area up to 

 a certain value and then decreases. The value of this maximum 

 velocity varies closely as the square root of the thickness. I have 

 no observation of facts which would be explained by the above 

 peculiarities; and the flakes of mica in stratified rocks are usually 

 too small for their application. However, it follows from my 

 experiments that thin flakes of mica of considerable area would 

 subside much more slowly than grains of sand of small area ; thus 

 we can easily understand why, as is so common, mica and fine sand 

 occur in separate layers. 



