19-2 K. II'. Sweats — The Formation of Dolomite 



evidence of limestones from upraised coral islands, lias made it 

 clear that in such cases of regional dolomitization tlie process 

 has gone on heneath the sea, and the only possible source of 

 magnesium consists in the salts in solution in sea water. In 

 the case of certain coral limestones such as those between 15 

 and 25 feet in the Funafuti bore, and others from elevated 

 fringing reefs at Christmas Island, Nine, etc., it is clear that 

 MgCOj has replaced CaC0 3 , up to 10-16 per cent, without 

 the production of definite crystals of dolomite. The shapes of 

 aragonite and calcite have persisted, in spite of the introduction 

 of MgC0 3 . Microscopical examination of limestones richer in 

 magnesia has shown that when dolomite crystals are developed 

 the process usually proceeds on definite lines. The structure- 

 less matrix of the limestone is dolomitized before the larger 

 crystals or the organisms. 



The organisms are attacked in a definite order, those whose 

 constitution was originally aragonite such as the corals, Hali- 

 meda, and gastropods being dolomitized before organisms built 

 of calcite, such as most of the foraminifera, Lithothamnion, 

 ecliinoderm spines, etc., while among the organisms built of 

 calcite, those like Lithothamnion in which a considerable 

 amount of organic matter is present are usually dolomitized 

 before organisms such as ecliinoderm spines in which little or 

 no organic matter is present. The process of dolomitization is 

 therefore selective, and most of the stages in the introduction 

 of MgC0 3 and of replacement of CaC0 3 by dolomite can be 

 followed in a series of rock sections of limestones in which the 

 change has taken place. Ultimately a dolomite may be formed 

 which is quite structureless, in which all traces of organisms 

 have disappeared, but that they were formerly present is reas- 

 onably certain from the study of the stages less complete 

 where traces of the outlines of the organisms remain as "dirt 

 lines" or "ghosts" defining their boundaries. No one wdio has 

 made a microscopic examination of such dolomitic limestones 

 can doubt that the change from a non-magnesian limestone to 



CD ~ 



a dolomite has taken place by a gradual introduction of mag- 

 nesian salts from the sea water involving a corresponding solu- 

 tion and removal of CaC0 3 . 



Furthermore, since in the formation of these marine lime- 

 stones the sea is the only possible source of magnesium, it fol» 

 lows that the dolomitization of the limestone followed so closely 

 on its formation as to be almost contemporaneous with it in a 

 geological sense. 



In this respect we may distinguish between these regional 

 "contemporaneous" dolomites and the local or subsequent dolo- 

 mites occurring generally along joints or mineral veins which 

 have been formed by the passage of magnesian solutions along 



