﻿138 PROF. E. W. SKEATS ON THE [Feb. I905, 



VIII. Summary oe Conclusions. 



1. liecent work on modern coral-reefs has extended our know- 

 ledge as to the chemical composition of the rocks of which they are 

 built up, especially as to the general absence of insoluble residue 

 in the limestones. 



2. The study of the relative proportions of the organisms com- 

 posing coral-reefs and the alterations which they undergo has shown 

 that corals generally play a subordinate part, and that calcareous 

 alga?, foraminifera, and other organisms form the bulk of the 

 rocks composing the reefs. 



I have applied this information in the examination of collections 

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



3. I find that many of the dolomites are devoid of insoluble 

 residue, and that where residue is present it can be generally 

 attributed, as in the raised reefs of Fiji, etc., to association with 

 contemporaneous volcanic rocks. 



4. Examination of thin sections of the rocks shows that the 

 limestones have undergone mineralogical and chemical changes 

 similar to those which have affected the limestones of modern 

 reefs, and that similar organisms are represented in the Tyrol 

 Dolomites, allowing for the loss of structure due to more complete 

 dolomitization. 



5. That the Schlern Dolomite of Southern Tyrol probably re- 

 presents Triassic ' coral-reefs,' using the term in the modern, more 

 extended sense. 



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

 that the general conditions favourable to their formation were : — 



(a) Shallow water between and 150 feet in depth, and corre- 



sponding to a pressure of 1 to 5 atmospheres. 



(b) The presence of carbon-dioxide in comparative abundance, 



causing the partial solution of the limestones and the 

 possibility of chemical interchange with the magnesium- 

 salts in sea-water. 



(c) Porosity of the limestones, allowing of the percolation of sea- 



water through the mass of the rocks. 



(d) Sufficiently-slow subsidence or elevation to render the 



change from calcite to dolomite complete. 



7. Locally, dolomite is deposited directly from solution in confined 

 areas or cavities in the rock, while some of the Raibl Dolomite 

 associated with gypsum was formed by the concentration of sea- 

 water in land-locked areas. 



