288 Geological Society. 



the relative proportions of the organisms composing these reefs, and 

 the alterations that they undergo, has further shown that corals play 

 a subordinate part in them, and that calcareous algaB, foraminifera, 

 and other organisms form the bulk of the rocks of the reefs. The 

 author has applied this information in the examination of collec- 

 tions from the much debated area of the Dolomites of Southern Tyrol. 

 The chemical examination of numerous specimens from the Schlern 

 dolomites of the Schlern, the Langkofl, the Marmolata, the Sella, 

 the St. Cassian district, the Eichthofen Keef, and numerous other 

 localities, is described, so far as relates to the proportions of lime 

 and magnesia and of insoluble residue. These results are compared 

 with similar analyses of limestones from lower and higher horizons. 

 Many of the dolomites are devoid of insoluble residue, and where 

 this is present it can generally be attributed, as in the raised reefs 

 of Fiji, etc., to the association with contemporaneous volcanic rocks. 

 The action of solution during the slow accumulation of deep-sea 

 limestones, and the presence of terrigenous material in organic 

 deposits forming near the shore, give to these limestones a much 

 larger proportion of insoluble residue. It is therefore probable that 

 the Schlern dolomites represent a Triassic ' coral-reef,' using the term 

 in the modern, more extended, sense. The author proceeds, on this 

 hypothesis, to suggest the conditions under which the dolomite may 

 have been deposited in the different parts of the area in question. 

 He next proceeds to the microscopical examination of specimens, 

 with a view to the study of the organisms contained in them. 

 While the limestones have undergone mineralogical and chemical 

 changes similar to those of recent coral-reefs, they contain a suite 

 of organisms comparable with those of the latter, when due allow- 

 ance is made for the loss of structure due to more complete dolo- 

 mitization. 45 per cent, of the sections yielded no trace of organic 

 structure. In the others, calcareous algae are the commonest fossils, 

 echinoderm-spines and lamellibranch-shells are not uncommon, 

 while foraminifera are occasionally found ; undoubted corals are 

 extremely rare. But reef-forming corals, gasteropoda, and some of 

 the calcareous alga? are built of aragonite, and hence are likely to 

 disappear under the operation of mineralogical changes ; while even 

 the more stable organisms with calcite-skeletons begin to lose struc- 

 ture, owing to the invasion of crystals of dolomite. 



With regard to the origin of the dolomite-masses, it is shown that 

 the conditions favourable to their formation were : — (a) shallow 

 water between and 150 feet, and corresponding to a pressure of 

 1 to 5 atmospheres; (6) the presence of carbon- dioxide in com- 

 parative abundance causing partial solution of the limestone, and 

 the possibility of chemical change with the magnesian salts in sea- 

 water; (c) porosity of the limestones facilitating the movement of 

 magnesian solutions ; and (d) sufficiently-slow subsidence or ele- 

 vation to render the change from calcite to dolomite complete. 

 Locally, dolomite is deposited directly from solution in confined 

 areas or cavities in the rock ; while the Raibl Dolomite, associated 

 with gypsum, was formed by the concentration of sea-water in land- 

 locked areas. 



