GEOLOGICAL CONCLUSIONS. 115 



minerals. Instead, therefore, of being extraneous minerals intro- 

 duced into the limestone rock, they are (or may be in some 

 instances) an essential part of its constitution. And they have 

 been separated from the general mass by a segregation of like 

 atoms, under well-known principles, while the rock was sub- 

 jected to an elevated temperature. The fluorid of calcium ap- 

 pears to crystallize without much heat ; but apatite and chondro- 

 dite are found mainly in granular limestones, which show, by 

 their crystalline texture, that they have been subjected either 

 to a very high temperature, or to one long continued of more 

 moderate degree. 



Lord Byron, of the Blonde, states that specimens of phosphate 

 of lime (apatite,) were actually collected on Mauki, of the Her- 

 vey Group, one of the elevated coral islands. 



VII. The cementation of coral sand along shores and beneath 

 the sea 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 

 covered in part by a smooth, solid crust, two or three lines thick, 

 and made of layers IFke stalagmite, which was formed by the so- 

 lution of lime from the surface by the rains, and its deposition 

 again on evaporation. 



The waters of the sea have been found to contain a small pro- 

 portion 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 car- 

 bonate of magnesia, this salt is largely present in some specimens 

 of the rock. The rock is hard (H. = 425), 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 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, carbon- 



* The magnesia in this analysis was directly determined, the lime being inferred 

 from the loss. 



