SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. lil 
probably rhombohedral, but their mutual interference during growth 
has prevented them from showing their characteristic shape. Many of 
the calcite crystals have dark muddy centers, a feature which is quite 
commonly met with in dolomite, but occurs very rarely in calcite. The 
type of alteration just described is not peculiar to corals, but is shared 
by all the aragonite organisms occurring in the coral rocks. Figure 5 
from Mango, at a height of 370 feet, is an example of a fragmental 
rock which has suffered such a change. 
Fig. 6. 
MaxatTeEa, 150-175 feet. The matrix of the rock consists of partially recrystal- 
lized “mud.” The cavities are lined with large scalenohedral crystals of calcite, 
many of which are very clearly zoned. X 80. 
The matrix was originally “mud,” but has now in part been changed 
to definite crystals of calcite. 
Most of the aragonite organisms such as corals, some molluscan shells, 
tunicate spicules, and Halimeda have also been recrystallized, the original 
borders of such as were present being represented by dirt lines. It will 
be seen that organisms such as Lithothamnion and echinid spines, whose 
skeletons consist of calcite, show no tendency to change, and for the 
most part are quite unaltered. 
Although when calcite is formed, it is most commonly found to crystal- 
