﻿Vol. 6 1.] DOLOMITES OF SOTTTHEKN TYROL. 129 



definite arrangement within the coral. What is still more inter- 

 esting, is the occurrence within the mass of the coral of extremes- 

 minute, perfectly-clear, doubly- terminated quartz-crystals, which 

 constitute about 1 per cent, of the rock (PI. XIV, fig. 2). 



When the dolomitization of a limestone is nearly complete, it is 

 noticed that the dolomite-crystals mutually interfere, so that the 

 crystals are mainly allotriomorphic. Among the Schlern rocks, 

 No. 7 affords a good example of a rock approximating to a dolo- 

 mite of theoretical composition, and containing both idiomorphic 

 and allotriomorphic crystals. 



With the production of a pure dolomite, the rock usually becomes 

 quite structureless and the crystals almost entirely allotriomorphic. 

 No. 56, from the Hexenfels, is a good example of this type of 

 dolomite. 



Dolomite-crystals are sometimes quite clear and transparent. 

 More generally, however, they are seen to contain cloudy matter ; 

 and this is generally orientated within the crystal, sometimes 

 constituting a dark centre, sometimes being arranged in one or 

 more zones parallel to the rhombohedral outline of the crystal. 

 These cloudy centres and zones may originate in several different 

 ways. 



Prof. Watts has shown, in the case of the dolomites of the 

 Carboniferous Limestone, 1 that this cloudy material is insoluble 

 matter caught up and regularly arranged during the growth of the 

 crystals. 



My own observations indicate four additional causes : — 



1. In some dolomites, staining with Lemberg's solution has 



shown that the cloudy matter is calcite. 



2. In other dolomites, especially those of theoretical composition, 



there is little doubt that the cloudy matter consists of finely- 

 divided dolomite, which does not stain with Lemberg's 

 solution. 



3. In some of the St. Cassian limestones the dolomites are zoned 



with cloudy chalybite, more or less altered to iron-oxide. 



4. Occasionally, these cloudy areas, when examined under a high 



power., have been seen to be due to numerous minute 

 cavities in the crystals. There is no direct evidence to 

 1 show whether these cavities are original, or due to subse- 

 quent solution of certain parts which, owing to their physical 

 state or chemical composition, were more susceptible to the 

 solvent action of sea- water. Their irregular distribution in 

 the parts of the crystals in which they occur is against the 

 latter view of their origin. 



Quite apart from the zoning in dolomite-crystals produced by this 



1 ' Geology of the South- Wales Coalfield, pt. ii : The Country around 

 Abergavenny ' Mem. Geol. Surv. (1900) pp. 34-36. 



Q. J. G. S. No. 241. k 



