﻿206 BE. H. C. SOEBY OX THE APPLICATION OF [May I908, 



consisted of fragments of bivalve shells, more than one-half of the 

 present solid rock must have been a subsequent chemical deposit 

 from solution, brought in by percolating water. It is manifest 

 that calculations in such cases are of little value. Deposits almost 

 entirely composed of oolitic grains of nearly equal size are very 

 suitable, and so are those composed of small joints of encritiites or 

 fragments of shells or corals worn into fairly equi-axed grains 

 of nearly equal size. Cases totally unfit for calculations are 

 common, in which the grains are of very unequal size, and the 

 spaces between the larger filled by smaller, as though not sorted by 

 a bottom-current. 



Sprudelstein, Carlsbad. — The amount of spaces between 

 the almost spherical grains, as determined by the camera-lucida 

 method, was found to be 44-3 per cent,, which corresponds to what 

 happens when deposited spheres are only slightly shaken. They 

 must have been soon filled with infiltrated material in such a 

 manner that all further settling was impossible. 



Recent da>posit, St. Helena. — What was given to me as 

 such many years ago, is composed almost entirely of rounded 

 grains of calcareous algae, nearly all of one size. The amount of 

 interspaces ascertained by the camera is 39-6 per cent. This 

 corresponds very closely with 40 per cent, in the case of shot or 

 sand well shaken. They are almost completely filled by infiltrated 

 calcite, and probably this took place at an early period in the 

 history of the rock, and its structure was thus made permanent. 

 This specimen is extremely interesting, because the percentage of 

 interspaces corresponds so closely with that observed in many older 

 rocks, down to the Silurian. 



Oolitic rocks. — It appears to me that a good deal remains to 

 be learned respecting the exact conditions under which our Oolitic 

 rocks were formed, since they differ so much from any deposits 

 associated with recent coral-reefs that I have been able to examine, 

 in which I have found only a very few oolitic grains of a kind 

 rarely seen in British rocks. These generally appear to have been 

 formed originally of calcite, a few of aragonite, and probably in 

 certain districts of a mixture of the two. Over a considerable area 

 in the Great Oolite, what was the original deposit is now a mere 

 wreck. Properly to explain all these variations would require 

 further researches of various kinds. For my present purpose, some 

 of these rocks are of especial interest — because the oolitic grains 

 can very fairly be looked upon as small spheres. In the first 

 place, I will consider cases in which there has been little change 

 since deposition. 



Oolites of the Lincolnshire district, etc. — I have two 

 excellent microscopical sections of a specimen given to me, and 

 said to come from near Grantham (see PI. XVII, fig. 1). The 



