﻿Vol. 6 1.] DOLOMITES OF SOUTHERN TYROL. 127 



such as Halimeda, are much less readily recognized in thin sections 

 of limestones in which this change has taken place. When, as not 

 infrequently happens, dolomitization of the limestones occurs, not 

 only are all traces of the organisms previously mentioned generally 

 obliterated, but the more stable organisms with calcite-skeletons, 

 like the echinodermata, the foraminifera, and most of the calcareous 

 alga?, begin to lose structure, owing to the invasion of crystals of 

 dolomite. In general, the more complete the dolomitization, the 

 more complete is the destruction of the organisms composing the 

 rock, until a perfectly-structureless, homogeneous dolomite may be 

 formed. In recent coral-limestones these changes can be traced 

 gradually in a succession of sections ; but the early stages in the 

 life-history of the Triassic dolomites of Southern Tyrol are closed. 

 All the rocks are altered. Where they are not dolomitized, they have 

 been recrystallized as the result of earth-movements ; and where 

 the rocks are undisturbed, dolomitization has been so complete that 

 many of the rocks are dolomites of theoretical composition. In 

 recent coral-limestones the rocks are often quite devoid of organisms ; 

 and it is therefore not surprising to find in rocks of Triassic age, 

 which are often more completely dolomitized than any recent lime- 

 stone, that organisms are always scarce, and in 45 per cent, of the 

 sections are apparently unrepresented. 



It will be seen that, with regard to the origin of the Schlern 

 Dolomite, the evidence afforded by a consideration of its organic 

 contents alone is inconclusive. The question is not whether the 

 Schlern Dolomite was originally a coral-limestone entirely composed 

 of corals, but* whether or no it was a coral-limestone of a more 

 common type (of which Funafuti and Christmas Island yield 

 examples), wherein calcareous algae and foraminifera bulk largely, 

 while corals occupy an important but relatively-subordinate position. 

 It is not possible to say definitely from this evidence that it was 

 originally a coral-limestone ; on the other hand, it seems to be 

 equally unsafe to conclude with some geologists that the comparative 

 absence of corals in the present condition of the rock proves that 

 they were never present in the original limestone. The solution of 

 the question as to the origin of the Schlern Dolomite should be 

 sought on other than palseontological evidence, which, so far as 

 it goes, does not preclude the possibility of the deposit having 

 originated as a coral-limestone. 



VII. MlNERALOGlCAL STRUCTURES AND CHEMICAL CHANGES 



in the Limestones and Dolomites. 



Mineralogical Structures. 



The examination of numerous thin sections from recent coral- 

 limestones l has served to show that, in general, three types can 



1 E. W. Skeats, Bull. Mus. Oomp. Zool. vol, xlii (1903) pp. 105 et seqq. j & 

 C. G. Cullis, 'Funafuti Eeport' Koy. Soc. (1.1)04) p. 404. 



