GEOLOGICAL CONCLUSIONS, 153 
is illustrated among all reefs, and is the process by which reef-rocks are 
formed, ‘The sea-water receives some carbonate of lime into solution, 
and again deposits it among the deposited sand and fragments which 
lie compacted together. The same process takes place among the 
beach sands and the drift heaps. ‘The eminences of drift sandrock at 
the Sandwich Islands were often covered in part by a smooth, solid 
crust, two or three lines thick, and made of layers like stalagmite, 
which was formed by the solution of lime from the surface by the 
rains, and its deposition again in evaporation. 
The waters of the sea have been found to contain a small proportion 
of free carbonic acid, which is sufficient to enable it to dissolve the 
carbonate of lime of the corals. 
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 carbonate of mag- 
nesia, this salt is largely present in some specimens of the rock. The 
rock is hard (H. = 4-25), and splintery in fracture, with the specific 
gravity 2-690. 
Carbonate of Lime, 2 - - - - 61°93 
Carbonate of Magnesia, — - - - - - 38:07 
Another specimen from the same island, having the specific gra- 
vity 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 coral sand, and coral mud from 
the islands, give no different composition, as regards the magnesia, 
from that for corals. The coral sand from the straits of Balabac 
afforded carbonate of lime, 98-26, carbonate of magnesia, 1:38, alumina, 
0-24, phosphoric acid and silica, a trace. 
We cannot account for this supply of magnesia except by referring 
to the magnesian salts of the ocean. It is an instance of dolomiza- 
tion, during the consolidation of the rock beneath sea-water, and 
throws light on this much-vexed question.* 
* While in the press, an article on the formation of dolomite from carbonate of lime 
came under the notice of the author, which affords a full demonstration of the view here 
suggested. It is by A. Von Morlot, and is published in the Naturwissenschaftliche 
Abhandlungen, herausgegeben von W. Haidinger, (4to. Vienna, 1847.) Von Morlot 
shows both by the frequent association of gypsum with dolomite, and by chemical experi- 
ment, that carbonate of lime and sulphate of magnesia, when together, undergo a double 
39 
