﻿Vol. 6 1.] DOLOMITES OF SOUTHERN TrEOL. Ill 



3. In general, such a limestone would not be deposited near a 

 non-calcareous shore-line, nor in the vicinity of volcanoes ejecting 

 ashes. 



These conditions are fulfilled, so far as I am aware, by only one 

 kind of deposit forming at the present day, namely in coral-reefs, 

 especially in those remote from great land-masses. I have already 

 drawn attention l to the high degree of purity of the limestones from 

 many of the upraised coral-islands of the Indian and Pacific Oceans ; 

 while, in the Report on the Atoll of Funafuti, 2 the chemical results 

 demonstrate that this typical atoll is composed entirely of limestone 

 and dolomite almost wholly free from insoluble residue. 



In considering the origins of limestones of geological antiquity, 

 we may say in general that : — 



1. A chemically-pure limestone has been formed under ' coral- 

 reef conditions. 



2. A limestone which, throughout its bulk, does not contain less 

 than 1 per cent, of insoluble residue, has been formed either in deep 

 water or as a detrital deposit, or has been laid down within the 

 range of the finer ejectamenta from a volcano. 



3. A limestone which in parts is pure, and in parts contains 

 residue, has probably been formed under ' coral-reef ' conditions, but 

 from time to time the area of deposition has been invaded either by 

 detrital or by volcanic material. 



We are now in a position to interpret the results of the analyses, 

 so far as the amount of residue which they contain may throw light 

 upon the mode of origin of the deposits. 



Wengen and St. Cassian Dolomites. 

 Area of the Schlern. 



From a consideration of the amount of residue alone, one might 

 be led to conclude that the rocks in this area represent deep-sea 

 deposits, since most of the analyses show well over 1 per cent, of 

 residue. One specimen, however, contains only m 2 per cent. If 

 the deposit were of deep-water origin, we should expect it to be thin 

 and well-bedded. We have to deal, however, with a mass of 

 unbedded dolomite approaching 3000 feet in thickness, laid down 

 contemporaneously with a much thinner series of shallow-water 

 limestones, marls, and tuffs, formed over the area of what is now 

 the Seiser Alp, to the north of the Schlern massif. The probability 

 is, then, that this mass of dolomite represents a lime- 

 stone rapidly formed in a subsiding area, and impreg- 

 nated with very finely-divided volcanic material from 

 submarine volcanoes lying to the north. 



This view is supported by the fact that, interbedded with the 

 Schlern Dolomite a few hundred feet above its base, there occurs a 

 deposit, a few feet thick, in which the volcanic fragments are bigger 



1 Bull. Mus. Comp. Zool. vol. xlii (1903) pp. 103-104. 



2 Roy. Soc. 1904, pp. 368 et seqq. 



