﻿Vol. 64.] QUANTITATIVE METHODS TO THE STUDY OF ROCKS. 185 



The usual size is 3 to 4 inches in medium sandstone. They are 

 not well seen in the coarse-grained Millstone Grit, in which, 

 however, they may be perhaps about a foot long. To judge from 

 these facts alone, it would seem as though the length varied some- 

 what as the velocity of the current necessary to wash along the 

 sand. The ripples seen on the sea-shore would in many cases 

 agree with this supposition. The mean length at Ryde was 3'7 and 

 at Sandown 6*9 inches. jN'ow and then I have come across some 

 which, so far, I am quite unable to understand. The most remark- 

 able were at the northern end of the Menai Strait, where there was 

 an extensive development of ripples some feet long. I was unable 

 to learn the exact conditions under which they were formed, but 

 still they make me think that other factors besides the size of the 

 grains of sand may occasionally play a very important part. The 

 current in the Menai is certainly strong, and the chief difficulty 

 is, not to understand why the ripples are long, but why they are 

 not washed away. It may be that the current along the bottom 

 itself was not particularly strong, while that higher up was much 

 greater, which determined the length of the ripples. 



VI. Yaetustg Size op the Geains. 



Another question of much importance in connexion with the 

 structure of sandstones and some limestones is the relative size of 

 the grains found mixed together. In some cases these are nearly 

 all of the same size, but in others, between certain limits, they 

 differ almost as much as possible. In many sandstones, although 

 there is considerable uniformity, numerous smaller grains occur in 

 the spaces between the larger. The exact cause of this occurrence 

 deserves more study ; but, as bearing on the question, I may refer 

 to the fact that a good many small grains may exist among the 

 coarse without producing any marked change in the angle of rest. 

 Many years ago I paid much attention to the general question, 

 and contrived a simple instrument for readily measuring the size 

 of the grains ; but it is only lately that the study of a different 

 class of rocks in Herefordshire has thrown light on the true nature 

 of the problem. Unfortunately, direct observations are difficult in 

 the case of consolidated rocks. 



Taking everything into consideration, the most important 

 general conclusion appears to be that more or less perfect 

 similarity in the size of the grains usually indicates a 

 sorting of the material by a current at the very bottom 

 of c omparatively -shallow water; whereas great irre- 

 gularity in the size indicates that the material was 

 deposited from much deeper water, in which there 

 was little current at the bottom, though a good deal 

 of current higher up. This is, of course, one of the most 

 important points in connexion with many rocks. 



