﻿Vol. 64.] QUANTITATIVE METHODS TO THE STUDY OF EOCKS. 203 



afterwards compressed it was reduced to 52. These facts show 

 clearly how very far small Hakes are from arranging themselves 

 in the smallest space, since, even when somewhat compressed, they 

 occupy more than the maximum for spheres arranged rectangularly. 



XIII. Segregation. 



Before further considering the cavities in sedimentary rocks, it 

 is desirable to point out the difference between two extreme forms 

 of alteration subsequent to deposition. One, which may be called 

 segregation from the outside, is when the cavities originally 

 existing between the ultimate particles have been more or less 

 completely filled, usually by carbonate of lime, introduced from 

 without from contiguous water or deposits, the original grains 

 being unchanged. The other may be called internal segre- 

 gation, and is when original material migrates from one place 

 to another, leaving empty spaces, and making other parts more or 

 less completely solid. As examples of the former, I may cite some 

 calcareous sandstones, and a case from St. Helena, where the 

 original rounded grains of corallines and shells are quite unchanged, 

 but the spaces between them filled with crystalline calcite ; and 

 as an example of the latter, a limestone from Binstead (Isle of 

 Wight), where the original fragments of shell have been com- 

 pletely removed, leaving empty cavities, and the carbonate of lime 

 has been transferred to the intermediate spaces. A combination 

 of both these changes is seen in many limestones. 



Coral-Reef Limestones. 



An analogous transfer from one part to another may result in 

 irregular cavities, and in a far more solid intermediate material 

 than the original deposit, conditions which are very characteristic 

 of those coral-reef rocks that I have been able to examine. 

 Through the kindness of the Trustees of the British Museum and 

 Dr. A. Smith AVoodward, I have been able to study carefully four 

 specimens from various depths in the boring through the reef at 

 Funafuti. In all of these there are empty spaces, in no way like 

 casts of decayed bodies, but of the most irregular shape : when 

 small, some may represent bubbles of gas, and when larger they 

 are of such irregular shape as to defy description. Their surface 

 is generally covered with small rounded protuberances. I cannot 

 explain the facts better than by internal segregation, causing the 

 material to collect into certain parts, so as to make them nearly 

 solid, and to leave other parts void, for lack of matter to make all 

 solid. 



These cavities are so unevenly distributed as to make it useless 

 to determine their amount from a small piece, but Dr. W. M. 

 Hicks kindly ascertained the specific gravity of the whole of each 

 specimen with the large balances at the Sheffield University, from 



