82 On Coral Reefs and Islands. 
Analyses of the coral limestone of the elevated coral island © 
Matea, by Prof. B. Silliman, Jr., have determined the singular 
fact that although the corals themselves contain very little car- 
bonate of magnesia, this salt is largely present in some specimens 
of the rock. The rock is hard (H.=4-25), and splintery in frac- 
ture, with the specific gravity 2-690. ; 
Carbonate of lime, . : : 61°93 
Carbonate of magnesia, ; ;  38-07* 
Another specimen from the same island, having the specific 
gravity 2°646, afforded 5:29 per cent. of carbonate of magnesia. 
The first was a compact, homogeneous specimen, and the other 
was partly fragmentary. Recent examinations of cora] sand, 
and coral mud from the islands, give no different composition, a8 
regards the magnesia, from that for corals. The coral sand from 
the straits of Balabac afforded carbonate of lime, 98-26, carbon- 
ate of magnesia, 1-38, alumina, 0-24, phosphoric acid and silica, 
a trace. 
We cannot account for this supply of magnesia except by re 
ferring to the magnesian salts of the ocean. It is an instance of 
dolomization, during the consolidation of the rock beneath sea- 
water, and throws light on this much vexed question. 
This subject is illustrated, and the view we sustain confirmed, 
by an article on the formation of dolomite from carbonate of lime, 
published in the Naturwissenschaftliche Abhandlungen edited by 
W. Haidinger, (4to. Vienna, 1847.) According to von Morlot, in 
this paper, Haidinger has recently shown both by the frequent 
association of gypsum with dolomite, and by chemical experiment, 
that carbonate of lime and sulphate of magnesia, when together, 
undergo a double decomposition, the magnesia taking the place of 
rt of the lime, and the excluded lime combining with the s 
phuric acid set free. The result is magnesian carbonate of limé, 
(dolomite,) and hydrous sulphate of lime, (gypsum,) the latter 
being separated, and either continuing in solution or solidifying; 
according to the amount formed or the proportion in the water. 
Von Morlot gives figures of specimens from different localities if 
which gypsum and dolomite are intimately associated ; and among 
them are some of fossil corals. | 
According to Haidinger, however, some heat is required for 
this result. Yet in the case of the coral rock and the compac 
magnesian limestones of our Western States, there is no evidence 
of the action of such heat; the subject therefore requires farther 
investigation. 
_ The circumstance of a chemical change going on between the 
carbonate of lime and magnesian salt, (for such a change, undet 
Se ee 
ae mile Ya tee el | 
