li>4 /'.'. W '. Skeats — The Formation of Dolomite 



at 1061 feet a dolomitic limestone occurred containing only 

 26-63 percent of MgC0 3 . 



Dr. Cullis found by microscopic examination that in each of 

 these lower parts of the core where the magnesian content was 

 noticeably lower than above or below them, that the rocks at 

 tliese places had been partially or completely dolomitizecl, but 

 subsequently their numerous cavities had been lined or filled 

 with later deposited layers, mainly of calcite but to some ex- 

 tent of dolomite, thereby reducing their magnesian content to 

 the values found.* ' 



The evidence from upraised coral islands. 



While the examination of the Funafuti bore cores was in 

 progress, the author was examining by the same methods, lime- 

 stones from various upraised coral islands in the Pacific and 

 Indian Oceans collected by Prof. Agassiz, E. C. Andrews, 

 Professor David, and Dr. C. W. Andrews. Some limestones 

 were found to consist mainly of aragonite, others of calcite, 

 and others of dolomite. Reference has been made above to 

 the fact that from some of the elevated fringing reefs of 

 Christmas Island, of Nine and elsewhere aragonitic limestones, 

 as in the shallow part of the Funafuti bore, were found to 

 contain from 8 to 13 per cent of MgCO, without any develop- 

 ment of the mineral dolomite. In the case of those upraised 

 coral islands in which dolomite was found, its zonal distribu- 

 tion measured by height above sea level at which it occurred 

 appeared at first to be quite capricious. The limestones of 

 certain of the islands have been dolomitized from top to bottom. 

 JNgillangillah and Vatu Vara belong to this class, and Yathala 

 and Kambara may also belong here. In several of the islands 

 no dolomite was found. Nine, Vavan, Tongatabu, Kiau, and 

 Guam are of this type. Non-magnesian limestones are asso- 

 ciated with dolomitic limestones in Christmas Island, Mango, 

 Namuka, and Eva. 



In these, dolomitization may recur at two or more horizons. 

 This is the case in Namuka, Christmas Island, and Mango. 



The shallow water origin of dolomite. 



Certain considerations in the distribution of dolomite in 

 coral limestones point strongly and the writer thinks unmis- 

 takably to its origin in shallow water. At Mango, Vatu Vara, 

 ISgillangillah, and Christmas Island, some of the most elevated 

 of the limestones have been dolomitized. This implies that 

 such limestones were the youngest and the only movements of 

 which we have evidence since their formation are movements 



* Cullis, Funafuti Report, London 1904, p. 410, fig. 44; p. 414, fig. 48. 



