SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 67 
Halimeda, etc., cemented with a large quantity of fibrous calcium car- 
bonate. After treatment with cobalt nitrate the slice was mounted, 
polished surface downward, on a glass slip, and the slice ground down 
till it was quite transparent. A cover-glass was then mounted on the 
section, and it was found that the corals, gastropods, and Halimeda 
were all stained red, the solid walls of the coral being less deeply 
stained than the septa. The echinid spine was quite unaltered, and 
no sign of staining was seen on the stalagmitic fibrous calcium car- 
bonate forming the cementing ground mass of the rock. This cement 
is thus shown to consist of fibrous crystals of calcite. It remained to 
test the prismatic crystals which were occasionally found in optical 
continuity with coral fibers. A slide showing this was prepared as 
above described, and it was then found that the septa of the coral 
and the “dark line” were stained a deep red, the solid parts of the 
coral and the prismatic crystals in continuity with the coral fibers 
were colored a lighter pink, while some clear crystalline calcite which 
filled other cavities was entirely unaffected. The prismatie erystals in 
continuity with coral fibers were thus shown to be aragonite. 
In discriminating between calcite and dolomite, there was, as a rule, 
not so much difficulty. The more idiomorphic character of dolomite 
crystals and their frequent zoning usually served to distinguish them, 
and when the two minerals occurred associated together, the higher 
refractive index of dolomite gave it a relief which was a useful diag- 
nostic character. In cases of doubt Lemberg’s' test was applied, with 
satisfactory results. 
The test consists in applying to the exposed surface of a thin section 
for about 5 to 15 minutes a solution containing a mixture of aluminium 
chloride and haematoxylin. Under these circumstances dolomite and 
brucite are unchanged, but a deposit of aluminium hydrate forms on the 
less stable calcite, and is stained a reddish purple by the haematoxylin. 
The staining solution is prepared by dissolving four parts of dry 
aluminium chloride in 60 parts of water, and adding six parts of log- 
wood (Haematoxylin campechianum). The whole was boiled and stirred 
for 25 minutes, and made up to the same bulk after filtration. It is not 
advisable to allow the staining solution to remain on the rock for more 
_ than 15 minutes, as dolomite is slowly acted upon, and also a thicker 
layer of alumina is deposited on the calcite, which is more easily peeled 
off and shrinks more on drying. After being stained, the section is 
1 Zeitschrift der deutschen geologischen Gesellschaft, 1888, Vol. XL., p. 357. 
VOL. XLII. — NO. 2. 2 
