﻿Vol. 64.] QUANTITATIVE METHODS TO THE STUDY OF ROCKS. 209 



described differences in the oolitic grains vary in different beds and 

 in different localities, but seem to extend over a considerable area. 



Limestones composed of fragments of calcareous 

 organisms. — When made up of joints of encrinites or of frag- 

 ments of shells or corals so nearly equi-axed that they may be 

 treated as imperfect spheres, their study leads to much the same 

 results as in the case of oolites. All my best specimens of Oolitic 

 age belong to the Coralline Oolite. That shown in PI. XYII, fig. 2 

 was found to have originally contained 43-3 per cent, of inter- 

 spaces, and another from Oxford 43*1. Yet another from Eiley 

 contained 53 •! per cent., this larger amount being probably due 

 to the fragments having too fiat a shape to be strictly comparable 

 with spheres. A Devonian limestone from Hope's Nose (near 

 Torquay), composed of small joints of encrinites, gave 45*6 per 

 cent. Leaving out the specimens from Tiley, the other three give 

 as a mean 44 per cent., or almost exactly the same as the 

 Sprudelstein of Carlsbad. Everything therefore agrees with the 

 supposition that the material was deposited rather quickly, and 

 not much shaken or pressed before the interspaces were filled with 

 infiltrated calcite. A specimen of the lower part of the Carboni- 

 ferous Limestone near Bristol, mainly composed of fragments of 

 encrinites, seems to have contained only 25 per cent, of interspaces, 

 as though they were not filled up with calcite until after the 

 material had been exposed to considerable pressure ; and a speci- 

 men from the Wenlock Limestone of Easthope (PI. XVIII, fig. 1) 

 contained only 17-7 per cent., which seems to have been due to 

 the still closer pressing-together of the angular and irregular 

 fragments of shells, corals, and encrinites. 



Taking, then, all these facts into consideration, the study of the 

 interspaces seems to indicate that, as might have been expected, 

 the effects of pressure are more marked in the older than in the 

 newer rocks. It reaches, however, its maximum in some Devonian 

 limestones associated with well -developed slaty cleavage. This is 

 notably the case in a portion of a thin bed composed of joints of 

 encrinites at Ilfracombe, in some i)arts of which nearly all traces 

 of the original interspaces have been obliterated. 



Magnesian Limestone. — The empty spaces seen with the 

 microscope in some specimens, as in that of Bulwell (near Notting- 

 ham), are really places left empty when the crystals of dolomite 

 were formed. In this they amount to 13'3 per cent. ; but the 

 whole history of the rock is too complex and difficult of under- 

 standing to make it desirable to do more than allude to it, as an 

 example differing entirely from most others that I have described. 



Cavities determined by the Boiling- Water Method. 



Although the percentage of cavities now existing in a rock may 

 throw little light on its original formation, yet it is of great interest 



p2 



