﻿208 DE. H. c. soEBY o:n^ the APPLicATioisr OF [May 1908^ 



rock. The mean of the two is 40*2, which is practically identical 

 with what occurs in the case of shaken shot and sand, and in the 

 recent rock from St. Helena. This seems to me a very interesting 

 conclusion, since it shows that right down to the Silurian Period 

 the infiltrated calcite was introduced while the rock was still what 

 would now be called ' recent.' In other cases it must have been 

 introduced when the rock was much older, or only partially up tO' 

 the present time. 



Modified oolites. — The freestones of Corsham (near Bath), 

 Minchinhampton, and Cheltenham differ remarkably from those 

 near Grantham and Ketton, inasmuch as the original oolitic grains 

 are now mere residues, and the present solidity of the rock is due 

 to the interspaces having been filled by infiltrated calcite. By the 

 camera-lucida method, I found that in the Cheltenham rocks the 

 interspaces originally amounted to 46 per cent., which does not 

 materially difl^er from what is found in the Sprudelstein of Carlsbad 

 (see p. 206). It seems to indicate a somewhat rapid deposition, and 

 little disturbance before the grains were permanently fixed by the 

 infiltration of calcite. As I pointed out in my address to the 

 Geological Society in 1879,^ there is good reason to believe that 

 the grains in a few of our oolitic rocks were originally deposited 

 as aragonite, but others as calcite ; and that the former have since 

 beeii changed, though not the latter (except in the case of the 

 Portland rocks, as described above). It seems quite possible that 

 some may originally have been a mixture of these two minerals, and 

 that, in the same manner as in the case of some fossil-shells, the 

 aragonite-portion has been removed and the calcite-portion left. At 

 all events, this would explain why some parts of the oolitic grains 

 have been removed, so as to give rise to their partly-decayed character. 

 Much more experiment is needed to ascertain the exact conditions 

 limiting the production of aragonite and calcite. Temperature 

 certainly, but evidently other things also have great influence. So 

 far, I have found very few oolitic grains in recent coral-reef rocks, 

 and these seem to be aragonite. The conditions under which our 

 characteristic oolitic rocks were formed appear to me to have 

 involved shallow water, perhaps rather warm, and highly charged 

 with carbonate of lime, which not only gave rise to the oolitic 

 grains, but also crystallized out between them very soon after they 

 were deposited. One specimen of the building-stone of Corsham 

 gave, by the boiling- water method, 25*6 per cent, as the amount of 

 cavities now existing ; but the microscopical structure shows that 

 this must have little to do with the original condition, although it 

 may have some connexion with the amount of cavities when the 

 oolitic grains were partly decomposed. I very much regret that I 

 did not measure the angle of rest of the material, since a knowledge 

 of the change in its value would probably have furnished valuable 

 information. Judging from the facts at my disposal, the above- 



^ Quart. Journ. Greol. Sol. vol. xxxv (1879) Proc. p. 84. 



