﻿"Vol. 64.] QUANTITATIVE METHODS TO THE STUDY OF EOCKS. 215 



XY. Concretions. 



The accurate, detailed study of concretions is a promising enquiry, 

 likely to yield interesting and unexpected results. Except iti the 

 case of the green spots in some slates, which I studied carefully many 

 years ago, I have unfortunately made but few accurate measurements 

 in the held ; and I must now rely on what can be learned from such 

 specimens as are in my collection, which, however, include those of 

 nearly all geological periods. It is desirable to divide them into 

 those formed before they were finally deposited, and those formed 

 in the rock after its deposition. The former division includes 

 oolitic grains, pisolites, and a few analogous objects ; and the latter, 

 more or less rounded bodies of very diverse character. 



Pisolites. 



The mean ratios of the three principal axes of the examples in 

 my collection of Oolitic age are 0*4, 1, and 1'33, no two being 

 equal, and the shortest being unusually short. This is largely 

 due to the flat nucleus. In a sandstone at Barlborough, near 

 ShefEeld, probably of Lower JS'ew-Eed-Sandstone age, the ratios 

 of the axes are 1, 1, and l'4l, and the well-developed grains are 

 remarkably uniform in size. They are composed of fine sand held 

 together by carbonate of lime, and their peculiar form was probably 

 due to rolling along the bottom. 



One very important fact connected with pisolites is that their 

 flatter surfaces are often inclined at various angles to the stratifi- 

 cation, as though deposited irregularly from a rather strong current. 

 Concretions formed after deposition usually have their longer axes 

 all nearly in the plane of stratification. 



Oolitic Grains. 



These are often remarkably uniform in size in each part of the 

 rocks, as though more or less sorted by a current of water. 



The Plane of Symmetry. 



So far as I can judge from the specimens in my collection, the 

 plane of symmetry of the rocks in three cases out of four coincides 

 with the plane of stratification, but in one-fourth of the cases it is 

 inclined at an angle varying from 5° to 10°. This is apparently 

 in no way due to chance, being uniform in all the specimens from 

 the same place. Those in my collection are not adequate to clear 

 up all the difficulties, which would require extensive work in the 

 field. The main question is whether the inclined plane of symmetry 

 was produced at the time of deposition, or long after. If produced 

 at the time, it seems to indicate that in that particular place the 

 bottom was not quite horizontal, but inclined at a considerable 

 angle, and that, although the layers of different quality lie in the 



