FORCHHAMMER ON THE FORMATION OF DOLOMITE. 51 



neralogists, it appears, will allow the combinations to be of value 

 only for dolomite that contains equal equivalents of carbonate of lime 

 and carbonate of magnesia, or, at least, a very simple equivalent-pro- 

 portion of the carbonated salts. But, according to a number of ana- 

 lyses I have made, carbonate of lime and carbonate of magnesia are 

 not combined in the compact dolomites in any simple equivalent-pro- 

 portion, which I have only found in the granular crystalline varieties. 

 One might therefore assume that the compact dolomite is a mecha- 

 nical mixture of carbonate of lime and carbonate of magnesia, whilst 

 the granular crystalline dolomite is a real double-salt [carbonate of 

 lime and magnesia] ; but I have found, especially in the compact do- 

 lomite from Faxoe, that this also contains a double-salt of equal atoms 

 of carbonate of lime and carbonate of magnesia, mixed with pure, or 

 nearly pure, carbonate of lime. When indeed I treated this dolo- 

 mite with acetic acid, it was partly dissolved, and there remained a 

 coarse white powder. The soluble part consisted of — 



Carbonate of lime 97* 13 



Carbonate of magnesia 2*87 



100-00 

 The insoluble portion, on the other hand, was composed of— 



Carbonate of lime 58*58 



Carbonate of magnesia 41 '42 



100-00 



This last proportion almost amounts to equal atoms of the two salts ; 

 but still the lime slightly predominates * . 



The well-known dolomite from Fullwell near Sunderland belongs 

 also to this group. It behaves in the same manner when treated 

 with acetic acid, a white granular powder remaining in large quan- 

 tity, the composition of which, however, I have not yet fully ex- 

 amined. And I think it very probable that the compact dolomites 

 are mechanical mixtures of carbonate of lime and a dolomite composed 

 of equivalent proportions of the two carbonated salts. 



The next question is of far greater importance, namely the geolo- 

 gical relations of the dolomite ; but I must here omit to notice some 

 of the most important phsenomena connected with the formation of 

 dolomite, as I am but slightly acquainted with Jthe dolomite for- 

 mation of the Alps, and can hardly assume that I am sufficiently 

 intimate with the disputed and very difficult questions connected 

 with Von Buch's dolomite theory. I intend, therefore, here to treat 

 of the compact dolomite only. 



In the course of this summer I have discovered dolomite in the 

 chalk-hill at Faxoe, where it occurs under conditions that are extra- 

 ordinarily illustrative. The Faxoe limestone [Faxoekalk] lies at 

 Stevnsklint between the writing-chalk, which is the lowest stratum, 

 and from which it is separated only by an inconsiderable clay-bed, 



* In order to be in exact equivalent proportions, there should be 54*4 carb. lime, 

 and 45'6 carb. magnesia. 



