﻿134 PKOF. E. W. SKEATS ON THE [Feb. 1905, 



at their summits ; some of their fringing-reefs, too, are dolomitized, 

 while the raised fringing-reefs along the coasts of the Red Sea are 

 also occasionally dolomitized. Dana was of opinion that magne- 

 sium was introduced into the rock from the waters of the lagoon 

 during concentration. Many lagoons, however, are quite open to 

 the sea, and concentration of the water is not possible in most of 

 them ; while, on the other hand, the outer parts of fringing-reefs 

 facing the open ocean are sometimes dolomitized. 



My view, then, is that the Schlern Dolomite originated first as a 

 limestone, composed of organisms, in a slowly-subsiding area. 

 Dolomitization of the limestone in superficial waters kept pace 

 with the slow subsidence, so that the whole thickness of 3000 feet 

 or more of rock was continuously and uninterruptedly converted 

 into dolomite during the Triassic Period. Some areas in the Tyrol 

 are only partly dolomitized, while others, like the Marmolata, are 

 for the most part undolomitized limestones. 



One important factor in the dolomitization of a limestone is the 

 time during which the limestone is submitted to the conditions 

 producing dolomitization. If it be true that dolomitization of sub- 

 merged limestones takes place in shallow water, then the extent to 

 which a limestone can be dolomitized will depend, other things being 

 equal, upon the length of time during which the limestone remains 

 in shallow water, that is, upon the rate of subsidence or elevation 

 of the mass. Complete dolomitization will only be effected under 

 stationary conditions in shallow water, or as a result of a very slow 

 subsidence or elevation; while, conversely, a limestone which is raised 

 or lowered comparatively rapidly, may undergo no dolomitization. 



The problem is certainly complicated by other considerations. 

 Among these, an important question is the extent to which the 

 rock is permeable to sea- water. The growing part of the reef just 

 below the surface is generally very porous, and admits of the free 

 penetration of sea-water, and sometimes the older part of the 

 limestone is also sufficiently porous to admit of the introduction of 

 water throughout its mass. Perhaps more generally, however, that 

 part of the structure upon which the living reef rests loses its 

 porosity, and is converted into a dense limestone by the filling-up 

 of the cavities by detrital material, or the fine mud produced by the 

 partial solution of the loosely-consolidated upper part of the rock. 

 Since the conversion of such a limestone into dolomite is effected 

 by an interchange between magnesian salts in sea-water and the 

 calcium-carbonate of the rock, it follows that the production of a 

 compact rock is one of the factors which shields a limestone from 

 dolomitization. Prof. Branner has adopted this view, in order to 

 explain the local character of the dolomitization of the Brazilian 

 reefs. 1 



I think that the varying power of sea-water to dissolve 

 limestones is another factor in the question of the formation of 

 dolomite. This power depends mainly upon the amount of carbon- 



1 Bull. Mus. Comp. Zool. vol. xliv (1904) p. 265. 



