380 



THE GEOLOGIST 



Although vre have given Mr. Hunt's paper only in a slightly condensed form 

 and almost in his own words, the portions we have omitted are not by any 

 means valueless, but sliould be read by those who are specially interested in 

 this cmious subject. We have abstracted sufficient to give the general reader 

 a clear notion of the state of investigation up to tliis point. 



In the " Canadian Journar' for May, IS 5 9, Mr. Hunt has published some ad- 

 ditional remarks upon tliis interesting subject in confirmation of his view "that 

 dolomites have been formed in sea-basins, from which the soluble salts of lime 

 have been completely separated, as sulphate or as carbonate by the agency of 

 alkaline carbonates, which afterwards gave rise to carbonate of magnesia," 

 which carbonate "appears capable, under certain conditions, of slowly combining 

 with carbonate of lime, and forming with it a double carbonate, which is 

 dolomite," 



Referi-ing to the Report under review, he says, " I have (there) show two 

 processes by which sediments of magnesian carbonate may be formed. Eirst, 

 by the action of solutions of bi-carbonate of soda on basins of sea-water, which 

 precipitate all the lime as carbonate, and then give rise to a soluble bi-carbonate 

 of magnesia ; and secondly, the action of bi-carbonate of lune on solutions con- 

 taining sulphate of magnesia. I have found that the presence of this salt 

 greatly increases the solubility of bi-carbonate of lime in water — bi-carbonate of 

 magnesia and sulphate of lime being formed by double decomposition. By 

 adding alcohol to such a solution, or by evaporating it at a gentle heat, 

 gypsmn is deposited, leaving the more soluble bi-carbonate of magnesia in 

 solution. 



" In the same way, alcohol separates gypsum from a mixed solution of bi- 

 carbonate of lime and sulphate of soda — an alkaline bi-carbonate remaining dis- 

 solved. 



" The subsequent evaporation in shallow lakes, or basins, of solutions of bi- 

 carbonate of magnesia, formed by either of the above-mentioned processes, must 

 give rise to deposits of hydrated carbonate of magnesia more frequently 

 mingled with carbonate of Hme, supplied by springs containing either bi- 

 carbonate of lime or cliloride of calcium. The hydi'ated carbonate of magnesia, 

 at 160 degrees C, and perhaps at a lower temperature, under pressure to pre- 

 vent the loss of carbonic acid, is converted into magnesite or anhydrous car- 

 bonate of magnesia ; but if carbonate of lime be present, the two combme to 

 form a double carbonate, which is dolomite, and may be separated from inter- 

 mixed carbonate of lime by the action of dilute acetic acid, at 32 degrees F., 

 wliich readily dissolves the latter, but attacks the dolomite more slowly. 



" I have found this union of the two carbonates to take place alike in the 

 presence of earthy and alkaline chlorides, sulphates, and carbonates, at tempe- 

 ratures between 130 and 200 degrees centigrade. A portion of the magnesia 

 is always, under these conditions, converted iiito magnesite, and may be par- 

 tially separated from the dolomite, by taking advantage of the fact that it is 

 less soluble in acetic acid at the temperature of 60 degrees E. than the double 

 carbonate. In nature, the combination must take place at the lowest possible 

 temperature, and one which is probably insufficient to produce the insoluble 

 magnesite. This, when once found, I have shown to have no tendency to unite 

 with carbonate of lime. 



" The application of these observations to the various conditions iii which 

 dolomites and magnesites are met with in nature, and especially to their asso- 

 ciation mth gypsum and anhydrite, is evident." 



Some further interesting remarks are added by Mr. Hunt, in his report, upon 

 Fish-manui-es ; and the volume is concluded by a paper by Lieut. E, D. Ashe on 

 the longitude of some of the principal places in Canada, as determined by 

 electric telegraph in the years 1856-7. 



