FORCHHAMMER ON THE FORMATION OF DOLOMITE. 0/ 



and the experiments establish that the proportion of magnesia in- 

 creases with the temperature. 



I cannot leave this part of my subject without remarking, that these 

 observations on the Faxoe Hill explain a phenomenon to which I have 

 previously drawn attention, yet without then being in a position to 

 explain it. The stratum, of which Faxoe Hill is a development, is 

 found over a very large portion of the Danish chalk, but in general 

 only with a thickness of two or three feet. At Faxoe it increases to 

 a thickness probably approaching a hundred feet, and it assumes 

 in its physical relations the character of a coral reef. The cause of 

 this local development is proved by the discoveries above given to be 

 owing to the calciferous springs bringing up such an abundant supply 

 of lime for the marine animals ; the warmth of the springs, moreover, 

 favouring perhaps the development of the latter. 



In many respects, the well-known dolomite from Fullwell near 

 Sunderland is analogous to the Faxoe dolomite. The same sphe- 

 roidal masses are found at both places ; but at Fullwell 1 have seen 

 them with a diameter of 4 feet. On the other hand the Fullwell dolo- 

 mite is distinguished by a form that is not found at Faxoe, viz. the 

 so-called *' honeycomb-stone," a dolomite that is full of holes that 

 are somewhat regularly arranged like the cells of a honeycomb. 

 These have been evidently produced by bubbles of disengaged car- 

 bonic acid gas, and are very important as they here occur in connec- 

 tion with spheroids formed by the action of springs, and therefore 

 explain the origin of the cavities, whilst on the other hand they form 

 the connecting link with the very cavernous dolomite that especially 

 occurs in the geological period now commonly recognized as the 

 Permian epoch, and to which the Ruevakke belongs. 



From these observations we can now also com.prehend why the 

 formation of gypsum is collateral with that of dolomite. Gypsum 

 having formerly been carbonate of lime from which the carbonic acid 

 was driven out by sulphuric acid, as is now usually accepted by geo- 

 logists, the carbonic acid so disengaged must, when water was present, 

 have redissolved a quantity of carbonate of lime ; and the reaction of 

 this solution on the sea-water must have formed dolomitic lime- 

 stones. Hereto belongs, for example, the singular dolomite of 

 Stipsdorf in Holstein, which is black and cavernous as a lava, and 

 contains somewhat worn specimens of the fossils of the brown-coal 

 formation, together with our common gravel-pebbles FRullestene] . 



Its composition is — 



Carbonate of lime 80*55 



Sulphate of lime 0'95 



Carbonate of magnesia 7*49 



Silica . 5-82 



Iron and argilia 2*83 



Coal, water, and loss 2*36 



100-00 



[T. R. J.] 



