154 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 15, 



are pure or mingled with carbonate of lime, are subsequently con- 

 verted into magnesite or magnesian limestone, may also be formed by 

 another reaction, which is independent of the formation of sulphate 

 of lime. I have already alluded to it, in my paper " On some points 

 in Chemical Geology," read before this Society on the 5th of January 

 last. Waters holding in solution bicarbonate of soda (a constant 

 product of the decomposition of felspathic minerals), when mingled 

 with sea-water, decompose the lime-salts first, the carbonate of lime 

 separating in a nearly pure state ; so that in lakes and limited sea- 

 basins there must result from this reaction waters that, like the 

 bitterns from which all the lime has been removed by evaporation 

 as sulphate (another source of gypsum), contain only salts of soda 

 and magnesia. The further action of solutions of bicarbonate of 

 soda upon such magnesian waters must give rise to bicarbonate of 

 magnesia, and, aided by evaporation, to precipitates of magnesian 

 carbonate — which, mingled with the carbonate of lime generally 

 accompanying the bicarbonate of soda in alkaline waters, would 

 afford the material for those great beds of magnesian limestone 

 which are independent of gypsum. (See Proc. Royal Soc. for May 18, 

 1858 ; Phil. Mag. vol. xvi. p. 379.) 



The points to which I wish to call attention in the present note 

 are, first, the formation of sulphate of lime and bicarbonate of 

 magnesia by the action of bicarbonate of lime upon a solution of 

 sulphate of magnesia, and their successive deposition in the forms 

 of gypsum and hydrous carbonate of magnesia during the process of 

 evaporation ; and secondly, the direct union, under certain conditions, 

 of this carbonate of magnesia with carbonate of lime to form a double 

 carbonate, which is dolomite. 



The application of these reactions to explain the formation of 

 dolomites and a great portion of the gypsums, both in marine and 

 freshwater deposits, will readily be made. The details of my experi- 

 ments will be found in part in the Report of the Geological Survey 

 of Canada for 1857*, and completed in the forthcoming Report 

 for 1858. 



3. On the Tertiary Deposits, associated with Trap-rock, in the East 

 Indies. By the Rev. Stephen Hislop. With Descriptions of 

 the Fossil Shells, by the Rev. S. Hislop ; and of the Fossil 

 Insects, by Andrew Murray, Esq., F.R.S.E. ; and a Note on the 

 Fossil Cypridje, by T. Rupert Jones, Esq., F.G.S. 



[Communicated by the President.] 

 (Plates V.— X.) 



In introducing the following descriptions of some fossils from Pen- 

 insular India, I shall confine my remarks to the rocks, volcanic 



* See also Am. Journ. Science, 2nd ser. vol. xxvi. p. 109. 



