SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 65 
carbonate from 15 different rocks showed a maximum difference of .93 
per cent and an average difference of rather less than .5 per cent. | 
MicroscopicaL MretHops.—In examining the thin sections of the 
limestones, my attention was directed not so much to the identification 
of the organisms (these have been described by Mr. R. L. Sherlock *) 
as to the determination of their mineral character, and the changes 
they and the matrix have undergone. It was as a rule sufficient for my 
purpose to be able to recognize corals, echinoderms, and molluscan 
shells, to distinguish between Halimeda and Lithothamnion among the 
calcareous alge, and to recognize a few of the more important Fora- 
minifera, such as Polytrema, Amphistegina, Heterostegina, Carpenteria, 
Orbitolites, Globigerina, etc. 
The precise identification of the mineral structure of the organisms, of 
the crystalline material subsequently deposited on them, and of the 
general character of the matrix, was not always an easy matter. 
In fibrous crystals it was often difficult to distinguish between arago- 
nite and calcite, while occasionally difficulties arose in differentiating 
calcite from dolomite. 
The characters usually relied on for the identification of aragonite are 
its occurrence in long prismatic crystals, its specific gravity (2.92), and its 
biaxial character. In contrast with these properties, calcite usually 
occurs in rhombohedra or scalenohedra, its specific gravity is 2.72, it 
has a strong rhombohedral cleavage, and it gives a well-marked uniaxial 
interference figure. Dolomite can, as a rule, be distinguished from cal- 
cite ? by its occurrence in simple unit rhombohedra often showing zoning, 
its specific gravity (2.9), its higher refraction, and the fact that its 
cleavage is usually less marked than that of calcite. As a rule, these 
tests were sufficient for the identification of calcite and dolomite, but gave 
no certain distinction for aragonite. Organic fragments were often 
found coated with an encrusting deposit of small radiating crystals, 
which were too small to allow of a specific gravity determination, 
while the interference figures given by these minute bundles of crystals 
were always unsatisfactory. When fibrous crystals were found in opti- 
cal continuity with the fibers of a coral, there was little doubt that the 
two substances were identical. In this connection it may be mentioned 
that a determination of the specific gravity of a coral from Niue, at a 
height of 80 feet, yielded a result of 2.81. 
Where no optical continuity could be traced between the deposited 
1 Bull. Mus. Comp. Zodél., 1903, Vol. XXX VIII. 
2 Renard, Bull. Acad., Belgique, May, 1879, Vol. XLVIL., No. 5. 
