GEOLOGICAL CONCLLSIONS. 307 



IX. FORMATION OF DOLOMITE, OR MAGNESIAN CARBONATE 

 OF LIME. 



Analyses of the coral limestone of the elevated coral island 

 Matea, by Prof 13. Silliman, junr., have determined the singular 

 fact that, although the corals themselves contain very little 

 carbonate of magnesia, magnesia is largely present in some 

 specimens of the rock. The rock is hard (H. .— 4), and 

 splintery in fracture, with a specific gravity 2 '690. It affords 

 on analysis, 38-07 per cent, carbonate of magnesia, and hence, 

 only 61*93 of carbonate of lime. 



Another specimen from the same island, having the spe- 

 cific gravity 2-646, afforded 5-29 per cent, of carbonate of 

 magnesia. 



The former was a compact homogeneous specimen, and the 

 latter was partly fragmentary. Recent examinations of coral 

 sand and coral mud from the islands give no different com- 

 position as regards the magnesia from that for corals, which, 

 as the analyses on page 75 show, contain very little or no 

 magnesia. The coral sand from the Straits of Balabac, 

 afforded Prof Silliman carbonate of lime 98*26, carbonate 

 of magnesia 1-38, alumina 0*24, phosphoric acid and silica 

 a trace. 



This introduction of magnesia into the consolidating 

 under-water coral sand or mud has apparently taken place 

 (i) m sea-waters at the ordinary temperature; and (2) with- 

 out the agency of any mineral waters except the ocean. 

 But the sand or mud may have been that of a contracting and 

 evaporating lagoon, in which the magnesian and other salts of 

 the ocean were in a concentrated state. It has been already 

 observed (p. 300), that this was probably the actual condition 

 of the elevated portion of the island of Matea, everything 

 about it looking as if it corresponded to the lagoon part of the 

 old atoll ; and also that the idea of the existence of mineral 

 springs there has no support in known facts. 



X 2 



