﻿Vol. 6 I.] DOLOMITES OF SOUTHERN TYROL. 131 



Formation of the Dolomites. 



The most important chemical problems in Southern Tyrol are, 

 perhaps, the following : — 



1. The mode of origin of those wide-stretching and thick masses 



of dolomite, of which the Schlern Dolomite is the most con- 

 spicuous example. 



2. A second problem is encountered in the attempt to explain 



the circumstances under which some areas, such as the 

 Marmolata massif, escaped dolomitization, while most of 

 the limestones of the same age are almost completely 

 dolomitized. 



3. The association of some of the dolomites of the Eaibl Beds 



with gypsum-deposits suggests a different origin for these 

 rocks. 



4. The local dolomitization of some of the St. Cassian limestones 



along cracks in the rock would seem to require a special 

 explanation. 



The early views of L. von Buch on the origin of dolomite were 

 formed as a result of the examination of the dolomites of the 

 Tyrol, and they involved the ascent of heated magnesium-vapour from 

 the outpourings of the Triassic volcanic rocks, which are found in 

 association with the dolomites. Modern chemists would not agree 

 with the chemical reactions involved in such a process, and, apart 

 from this, the distribution of the dolomites has no causal relation 

 to the distribution of the volcanic rocks. 



The analysis by Silliman, of a dolomitized limestone from the 

 raised coral-island of Makatea, led Dana to propound the view that 

 sea-water constituted the source of the magnesium, and that it 

 was probably introduced from the concentrated waters of the 

 lagoon. This view forms the basis of the modern explanations of 

 the formation of such dolomites. 



In some of the Tyrol limestones examined by me, magnesium- 

 carbonate is present in the rocks, in varying amounts up to 6 

 or 7 per cent., without leading to the formation of visible dolomite- 

 crystals. In a paper published in 1903, 1 I quoted analyses 

 from rocks at Christmas Island and elsewhere, in which lime- 

 stones containing over 11 per cent, of magnesium-carbonate were 

 similarly devoid of visible dolomite. Following Dana, I believe 

 that the magnesium is introduced from the sea-water, and is 

 absorbed by calcite up to 10 to 15 per cent, of its mass without any 

 visible change of form, but above that amount the stable compound 

 dolomite crystallizes out. 2 A similar result was obtained on ana- 

 lysing specimens from the upper part of the Funafuti borings.' 



1 Bull. Mus. Comp. Zool. vol. xlii (1903) p. 102. 



2 Mr. L. J. Spencer has described somewhat analogous behaviour in the 

 mutual relations of copper and silver-iodides, Miu. Mag. vol. xiii (19U1) 

 pp. 43-44. 



3 J. W. Judd, 'Funafuti Report ' Roy. Soc. (1904) pp. 373 et seqq. 



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