GEOLOGICAL CONCLUSIONS. 357 



carbonate of magnesia, magnesia is largely present in some 

 specimens of the rock. The rock is hard (H. == 4), and splint- 

 ery 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 mag- 

 nesia. 



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 99 show, contain very little or no 

 magnesia. The coral sand from the Straits of Balabac, af- 

 forded 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 (1) 

 in sea waters at the ordinary temperature ; and (2) without 

 the agency of any mineral waters except the ocean. But the 

 sand or mud may have been that of a contracting and evap- 

 orating lagoon, in which the magnesian and other salts of the 

 ocean were in a concentrated state. It has been already ob- 

 served (p. 347), that this was probably the actual condition 

 of the elevated portion of the island of Matea, every thing 

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

 the old atoll ; and also that the idea of the existence of min- 

 eral springs there has no support in known facts. 



X. FORMATION OF CHALK. 



The formation of chalk from coral is known to be ex- 

 emplified at only one spot among the reefs of the Pacific. 



