﻿Vol. 6 1.] DOLOMITES OF SOUTHERN TYROL. 135 



dioxide dissolved in the water, or liberated on the decay of the or- 

 ganisms composing the rock. Probably the interchange of magnesian 

 for calcium-salts is most readily effected at the moment of solution of 

 the calcium-carbonate of the rock. If this be so, the carbon-dioxide 

 slowly liberated on the decay of plants and animals would help to 

 determine the introduction of magnesium. We should, in that case, 

 expect that unstable forms like the corals would be more rapidly 

 affected than organisms the skeletons of which are built of calcite; and 

 among the calcite-forms we should expect those to be most dolomitized 

 which contain the largest quantity of organic matter in their tissues. 

 Examination of thin sections of coral-limestones affords evidence in 

 favour of this view, since aragonite-forms, like the corals, and 

 Halimeda among the calcareous algse, contain much organic matter 

 in their tissues, and are the first organisms to suffer dolomitization 

 and loss of structure. Among the calcite-organisms, too, we find 

 that forms such as foraminifera and echinoderm-spines, containing 

 little organic matter in their skeletons, resist disintegration and 

 dolomitization longer than Lithotliamnion and Lithophyllum among 

 the calcareous algae. 



So far, the evidence given of the distribution of dolomite in lime- 

 stones of organic origin points to the change having taken place in 

 shallow water, while contributing causes are porosity of the 

 limestone, slow upheaval or subsidence, and the presence 

 of carbon-dioxide in the water, to serve as a solvent for 

 part of the limestone. What, however, are the precise conditions 

 governing the change from limestone to dolomite in the case of 

 great rock-masses, and the exact chemical reactions which take- 

 place, it is very difficult to say. Chemists are not all in agreement 

 as to the relative solubilities of the two carbonates in carbonated 

 water, and it may be that the reactions which take place in pure 

 water are modified in sea-water. There is, however, considerable 

 evidence to show that under atmospheric pressure calcium- 

 carbonate is more soluble than magnesium-carbonate. Simi- 

 larly, there is no doubt that, when dolomite is subjected to fresh 

 water containing carbon-dioxide at a pressure of about 5 atmo- 

 spheres, the magnesium- carbonate is dissolved and the calcium- 

 carbonate remains almost unaffected. This, indeed, was the process 

 formerly employed in the production of Epsom salts from magnesian 

 limestone. It would appear to follow, then, that at some pressure 

 between 1 and 5 atmospheres carbonated water will dissolve the 

 two carbonates in molecular proportions, and under saturated con- 

 ditions the double carbonate, dolomite, may be deposited. In this 

 connection it may be stated that Mr. Stanley Gardiner has quoted 

 experiments showing that dolomite is more insoluble in carbonated 

 water than either calcium-carbonate or magnesium-carbonate alone. 1 

 The change from calcite to dolomite involves a considerable shrinkage 

 in the bulk of the rock, amounting to about yyth of the original 

 mass. In view of this contraction, it is quite in accordance with 



1 Proc. Carab. Phil. Soc. vol. ix (1898) p. 496. 



