SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 109 
3, which consists of a section of a “coral sand” from the lower inland 
cliff of Christmas Island. The fibrous crystals of the matrix which 
radiate from adjacent organisms meet along straight lines and give a » 
polygonal appearance to the rock section. This deposit of fibrous cal- 
cium carbonate was formed from solution probably on a beach between 
high and low tide marks. It could only be identified with certainty 
as calcite after the application of Meigen’s staining method described 
above. After boiling with cobalt nitrate solution, it was noticed that 
Fie. 4. 
MaxkaTea. Third terrace. The substance of the coral has been replaced by 
crystals of calcite. The coral walls are represented by “dirt lines,” while the 
“ mud-filled” cavities have been partially recrystallized. Many of the calcite 
crystals have dark centers. X 30. 
the coral and deposited aragonite crystals, as well as Halimeda and 
the gastropod, with included aragonite, had been stained, while the 
echinid spine, Orbitolites, and Lithothamnion, together with the fibrous 
crystalline matrix, remained quite unaffected. 
In the newer rocks, of which the first three illustrations may be taken 
as examples, it will be noticed that the skeletons of all the organisms 
remain practically unaltered. When, however, older rocks are examined, 
it ig seen that some of the organisms no longer have a fresh appearance. 
The tendency of the unstable form aragonite to pass over to the more 
