﻿136 PEOP. E. W. SKEATS ON THE [Feb. I905, 



the well-established 'principle of least work' to find that 

 this change is facilitated by an increase of pressure. 



Translated into depths, the limits may be expected to lie between 

 the surface and 150 feet below it, and probably nearer the upper than 

 the lower limit. The solid matter in solution in sea-water consists 

 of 78-32 per cent, of sodium-chloride, 1*69 per cent, of potassium- 

 chloride, 9*44 per cent, of magnesium-chloride, 6*40 per cent, of 

 magnesium-sulphate, 3'94 per cent, of calcium-s'ulphate, and traces 

 of bromine, iodine, etc. What we require to know are the conditions 

 under which the magnesium-chloride and sulphate present in such 

 considerable amount in sea-water will react with the calcium- 

 carbonate dissolved by carbon-dioxide and in contact with the mass 

 of the limestone. Dr. C. Element's experiments l demonstrate that 

 this reaction can take place under pressure, and at a temperature 

 of about 100° C. He subjected calcite, aragonite, and aragonitic 

 organisms to water saturated with sodium-chloride and magnesium- 

 sulphate, enclosed under pressure in a sealed tube, and heated for one 

 or two days at a temperature of 90° to 100° C. The resultant action 

 was a selective one. The calcite took up only a trace of magnesium- 

 carbonate, aragonite absorbed 38 per cent., and corals and other 

 aragonitic organisms over 41 per cent. The chemical instability of 

 the aragonite was here the determining factor in the rapidity of the 

 interchange. It is possible that, if the calcite had been exposed to 

 the conditions of the experiment for a much longer period, the 

 introduction of magnesium-carbonate would have proceeded farther, 

 and probably the action would have been accelerated if the calcium- 

 carbonate had been brought partly into solution by carbon-dioxide. 

 The high temperature no doubt quickened the chemical changes, 

 which probably proceed more slowly at the temperature of ordinary 

 sea-water. 



While, in general, dolomitization takes place as a result of 

 the interchange of magnesium- for calcium-carbonate, the question 

 arises whether it is ever deposited directly from solution. Perhaps 

 most chemists are unfavourable to this view of its formation, and 

 direct experiments bearing to some extent on this question have in 

 general produced negative results. Dr. Sorby found that magnesite, 

 and not dolomite, was formed by the action of magnesium-sulphate 

 upon calcium-carbonate under high pressure. There is, however, 

 mineralogical evidence in favour of the view of direct deposition 

 of dolomite. The lining of calcite-crystals by an outer zone of 

 clear dolomite, described by me 2 as occurring in sections from 

 Mango (Fiji), and also noticed in some of the sections of Tyrol 

 dolomite described above, is difficult of explanation, except on 

 the hypothesis that the dolomite was deposited from solution in 

 optical continuity with the calcite. It sometimes happens that 

 one crystal is built up of successive zones, alternately calcite and 

 dolomite. As these crystals are but rarely recognized in thin 

 sections, it is clear that they were not formed under the conditions 



1 Tscbermak's Min. & Petr. Mitth. n. s. vol. xiv (1895) pp. 531 et seqq. 



2 Pull. Mus. Comp. Zool. vol. xlii (1903) pp. 117-18. 



