TRANSACTIONS OF THE SECTIONS. 83 



from the Lower Lias by not more than 25 feet of strata, of which the Marlstone forms 

 the greater part. 



Notes on some Points in Chemical Geology. 

 By T. Sterry Hunt, F.R.S., of the Geological Survey of Canada. 



Dolomites and Gypsum. — Mr. Sterry Hunt has shown, from the mode in which dolo- 

 mites occur and from the phenomena presented by their associated fossils, that these 

 magnesian rocks cannot have been formed by the alteration of pure limestones, so that 

 the theories of Von Buch and Haidinger, proposed to explain their formation, are 

 really in nowise applicable. He has further shown, that in the famous experiment 

 suggested by Haidinger and performed by Von Morlot, who asserted that by the 

 action of sulphate of magnesia, in presence of water in an excess of carbonate of lime, 

 at 200° C. under pressure, there is formed sulphate of lime and a double carbonate of 

 lime and magnesia, the fact has been overlooked that in reality no double carbonate 

 is obtained, but only a mixture of anhydrous carbonate of magnesia with carbonate 

 of lime, and consequently not a dolomite, which is a chemical compound of the two. 



In Marignac's modification of Von Morlot's experiment, where the chloride is sub- 

 stituted for the sulphate of magnesia, Mr. Hunt finds that a variable portion of this 

 double carbonate is really formed, and remains mingled with the excess of carbonate 

 of lime and anhydrous carbonate of magnesia, which is also a result of the reaction as 

 before. Charles Deville's late experiments, in which fragments of limestone were im- 

 pregnated with magnesian solutions, and heated at the ordinary pressure, with forma- 

 tion of soluble lime-salts and magnesian carbonate, are but imperfect repetitions of 

 Von Morlot's and Marignac's processes, and none of these are applicable to the great 

 majority of cases in which pure and magnesian limestones are associated in such ways 

 as to show that they have been successively deposited from water, the latter sometimes 

 enclosing pebbles and fossils of pure carbonate of lime. 



Mr. Hunt proceeds to show that, when mixtures of amorphous hydrated carbonate 

 of magnesia with carbonate of lime are heated under pressure to a temperature of 

 300° to 400° F., direct combination ensues, and dolomite is formed ; and he gives 

 reasons for supposing that this combination may take place slowly at much lower 

 temperatures. 



It was, however, necessary to find a source for the magnesian carbonate which had 

 formed these magnesian sediments, and here Mr. Hunt has signalized a remarkable 

 and hitherto undescribed reaction, by which carbonate of lime decomposes sulphate of 

 magnesia, not with the aid of heat and pressure as- in Von Morlot's experiment, but at 

 the ordinary temperature. When a solution of bicarbonate of lime is mingled with a 

 liquid containing sulphate of magnesia, a double decomposition takes place, and by 

 evaporation at temperatures between 90° and 180° F., the lime is deposited in the 

 form of gypsum, a very soluble bicarbonate of magnesia remaining dissolved, which 

 is precipitated by further evaporation. If we conceive the carbonate of lime to be 

 furnished by springs falling into a closed lake or basin, the carbonate of magnesia 

 would be precipitated in a state of mixture with carbonate of lime, thus giving the 

 elements of the dolomite which is always associated with stratified gypsum. 



Mr. Hunt has further shown that, by the action of waters containing alkaline carbo- 

 nates upon sea-water, the lime is first precipitated, and at length there is formed a 

 solution of bicarbonate of magnesia. To this agency he ascribes the vast deposits of 

 magnesian rocks which exist independent of gypsum, and which sometimes contain 

 an excess of carbonate of magnesia over that required to form dolomites, or lime being 

 absent, are magnesites. 



The part which carbonate of soda has played in giving rise to carbonates of lime and 

 magnesia must, according to Mr. Hunt, have been very important in former periods. 

 The source of this has been the decomposition of felspar, which, in being reduced to 

 clays, have lost the whole or a part of their soda in the form of silicate, which, converted 

 into carbonate by the carbonic acid of the atmosphere, is now represented by the sea- 

 salt of the ocean and the carbonates of lime and magnesia of the rocky strata. Clays 

 and argillites are unknown in the vast thickness of crystalline rocks which constitute 

 in Canada the Laurentian system, lying beneath the Lower Silurian series. In these 

 oldest rocks, the alumina exists in the form of felspar, in great part with a base of 

 soda ; but in the Silurian rocks, when altered, aluminous silicates abound, such as 



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