﻿Vol. 6 1.] DOLOMITES OP SOUTHERN TYROL. 137 



governing the general dolomitizafcion of the mass of the rock. It 

 may be that they were formed in cavernous parts of the rock, more 

 or less shut off from the free passage of the sea-water ; and, indeed, 

 these crystals are sometimes found lining the walls of cavities. 

 Dr. C. G. Cullis, in the mineralogical report on the Funafuti boring, 1 

 describes and figures a somewhat analogous case, of the deposition 

 of the two minerals in alternating coats lining the walls of cavities, 

 in a way that simulates on a microscopic scale the appearance of 

 the structure of an agate. 



It would appear from this that the composition of the solution 

 or its physical state varied from time to time, in such a way as to 

 lead to the alternate deposition of dolomite and calcite. Possibly, 

 in these restricted areas, the sea-water becomes saturated with 

 calcite and the double carbonate in turn, and the conditions of 

 equilibrium may resemble those investigated by Prof. Van 't Hoff and 

 others in connection with the Stassfurt salts and similar deposits. 

 Besides this peculiar and local formation of dolomite by direct 

 deposition from solution, in the Tyrol we have to consider two 

 other modes of occurrence of dolomite in some way resembling 

 that just described. Some of the St. Cassian Limestones which 

 immediately underlie the massive Schlern Dolomite of the Sella and 

 Sett Sass, for example, contain in general only a small amount of 

 magnesium-carbonate and usually show no visible dolomite. 



In both these areas, however, microscopical examination shows 

 that in places certain parts of the groundmass of the rock contain 

 very minute dolomite-crystals. These are generally confined to 

 the neighbourhood of some crack in the rock. No doubt, in this 

 case, carbonated water, passing through the Schlern Dolomite above, 

 dissolved some magnesium-carbonate, and, traversing the cracks 

 in the St. Cassian Limestone below, deposited its magnesium-car- 

 bonate as small crystals of dolomite. Occasionally, these minute 

 crystals are zoned with layers of calcite, and in one case with the 

 isomorphous carbonate chalybite. 



In the Eaibl Beds overlying the Schlern Dolomite local dolo- 

 mites are found, and these are sometimes intimately associated 

 with beds of gypsum, in such a way as to leave little doubt that 

 movements of upheaval during the Baibl period led to the enclosure 

 of limited areas or lagoons, and caused the concentration of the sea- 

 water and deposition of the calcium as gypsum and the magnesium 

 as dolomite. 



We have then, within the area of Southern Tyrol, to 

 deal both with local aud regional dolomitization. The 

 mode of occurrence of some of the dolomites of the Raibl Beds, and 

 the very partial and local dolomitization of the St. Cassian Limestones, 

 makes it easy to suggest a probable explanation of their origin. 

 The great problem of the origin of the Schlern Dolomite is, as has 

 been seen, a more complex one, and a complete solution of all the 

 difficulties will only be found when the chemical problems involved 

 have been more fully investigated. 



1 Koy. Soc. (1904) p. 410 & fig. 44. 



