200 F. W. Skeats — The Formation of Dolomite. 



and if the rate of subsidence, of a coral limestone, for instance, 

 exceeds a given value, little or no replacement may take; place, 

 since the limestone will then subside too rapidly below the 

 favorable zone. 



Porosity too may well be an important factor in the process 

 since a porous limestone will be readily permeated by sea 

 water and become dolomitized if other conditions are favorable 

 while a relatively dense and impervious limestone may remain 

 undolomitized. 



Relation of the distribution of dolomite to the formation of the 

 atoll of Funafuti. 



The author regards the evidence of dolomitization of fring- 

 ing reefs of coral islands, the occurrence of dolomite immedi- 

 ately below phosphate beds, on the hill tops of Christmas Island, 

 the rise in the magnesian content of the limestones of the Funa- 

 futi bore between 15 and 30 feet, as definite and strong evidence 

 of the shallow water origin of dolomite in coral islands. It is 

 claimed that this view is consistent with the chemical evidence 

 quoted above of the reversal of the solubilities of calcium and 

 magnesium carbonates iu carbonated water between the pres- 

 sures of one and four atmospheres. In addition, the evidence, 

 cited above from more ancient dolomites showing their intimate 

 associations with independent evidence of shallow water such 

 as fossils, current bedding, conglomerates, and oolitic structures, 

 is so consistent and so in accordance with the evidence from 

 modern coral limestones, that the author takes the view that 

 wherever a "contemporaneous" or regional dolomite is found to 

 occur, it may be regarded as having originated in shallow 

 water. If this be granted, it follows that such upraised coral 

 islands, like Ngillangillah now 510 feet high, and Vatu Yara 

 now 1030 feet high, which are dolomitized from top to bottom, 

 must have originally been formed of shallow water limestones 

 accumulated by subsidence to at least 500 and 1000 feet respec- 

 tively before elevation set in. The atoll of Funafuti whose 

 surface is practically at sea level must also have been built up 

 of shallow water limestones accumulated during subsidence, 

 which must have amounted to about 1100 feet at least since the 

 cores from 635 feet to 1114 feet consist entirely of limestones 

 which have passed through the process of dolomitization. 



In the discussion, therefore, of this chemical and miner- 

 alogical problem, apparently so remote, on first consideration, 

 from the question of the origin of atolls, there is brought 

 forward independent testimony in favor of the validity of 

 Darwin's theory of subsidence for the formation of the atoll of 

 Funafuti as well as of several upraised coral islands. This 

 testimony is advanced with the more confidence since it is in 

 agreement with the evidence of the organisms and of structures 

 set out in a preceding paper. 



