SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 107 
rectly from solution occurs much less frequently than that formed by 
the recrystallization of a calcareous “mud.” 
Another point of difference is that erystals deposited from solution ' 
grow from the walls of the cavity, and if they entirely fill it the crys- 
tals meet in the center in a serrated line, resembling the ‘comb 
structure” of a mineral vein. In a clarified “ mud” no such definite 
arrangement of the crystals can be traced. A study of thin sections 
of rocks from coral islands seems to show that when crystallization 
Fie. 2. 
Nive, 200 feet. Some cavities in the coral are filled with “mud,” while others 
have recrystallized. Aragonite has formed on the coral wall and round the tubes 
of boring alge, but calcite fills the remainder of the cavity. The spicular char- 
acter of the coral is well seen. X 80. 
occurs, either of a “mud” or from solution, calcite is in general the min- 
eral which is first formed. Aragonite has been found almost exclusively 
as a lining to the cavities of corals whose spicular structure seems favor- 
able to its formation. Figure 2 represents a section of a reef forming 
coral from the third terrace of Niue at a height of 200 feet. 
Some of the “ mud-filled” cavities remained unaltered, but others have 
been recrystallized. In the more central parts of these cavities the 
“mud” has erystallized as a granular mosaic of calcite, except round 
the tubes of boring alge which are surrounded with radiating crystals, 
