﻿Vol. 64.] QTJAN'TITATIVE METHODS TO THE STUBY OF EOCKS. 207 



nearly spherical grains are quite hard, and the interspaces almost 

 entirely empty, containing only a few small crystals of calcite. 

 The volume of these interspaces, as determined by the camera- 

 lucidn method, was found to be 25*3 per cent., which, allowing for 

 slight compression, corresponds remarkably well with 25-95, the 

 theoretical minimum for spheres. It should appear therefore, 

 that, although mere shaking will not cause grains of shot to arrange 

 themselves in the least volume, pressure (and perhaps earthquake- 

 vshocks) acting through geological periods will produce this result ; 

 but of course not, if the interspaces had been filled by infiltrated 

 calcite at an early part of the history of the rock. 



I have examined some analogous rocks from the same part 

 of England by the boiling-water method. The building-stone of 

 Ketton was found to contain 29-7 per cent, of empty spaces, and 

 another specimen from near Stamford 36"3 per cent. This latter 

 was extremely soft when obtained in the quarry, and the grains 

 themselves may not be solid. The excess over 26 per cent, may, 

 therefore, not be due to the grains not having accommodated them- 

 selves to the least volume, but to the different manner in which 

 the interspaces were determined, those in the grains themselves 

 having no influence in the case of the Grantham specimen. 



Portland oolite. — Determined by the camera-lucida method, 

 the Portland building-stone was found to contain 23-6 per cent, of 

 interspaces, only to a small extent filled with infiltrated calcite; 

 but it can be seen that the grains have been to a slight extent 

 pressed together, so as to explain why the interspaces are less than 

 26 per cent. When determined by the boiling-water method, those 

 now empty were found to amount to 22 per cent., which smaller 

 amount as thus determined is no doubt due to infiltrated calcite. 

 It is, however, only particular portions of the Portland rocks that 

 are at all suitable for the purpose in hand ; for, in some, the oolitic 

 grains have been changed since deposition in an unusual and 

 remarkable manner, especially when near the Dirt-Bed. 



Oolitic beds in the Carboniferous Limestone of 

 Clifton, near Bristol. — In one excellent case the original 

 interspaces must have amounted to 40 per cent., but these are 

 now filled by infiltrated calcite. This agrees with the supposition 

 that this infiltration occurred before the deposit was subjected to 

 much pressure, though in another specimen belonging to my friend 

 Mr. T. S. Cole, and photographed by him, the interspaces had been 

 reduced to 32*6 per cent., or even less. 



Oolitic bed in the Wenlock Limestone, near West 

 Malvern. — Determined by the camera-lucida method, the original 

 interspaces were found to have been 39*6 per cent. ; and thus, like 

 the above-described Carboniferous Limestone, they must have been 

 filled by infiltrated calcite at an early period in the history of the 



Q. J. G. S. No. 254. p 



