798 A TREATISE ON METAMORPHISM. 



niiitiiN ok dolomite. 



Dolomite is calcium-magnesium carbonate (CaC0 3 .MgC0 3 ). In consid- 

 ering the origin of dolomite the questions immediately arise: Are magnesian 

 limestones and dolomites originally deposited as such, or are they the results 

 of replacement of calcium by magnesium! and, if the latter, how and when 

 did the process take place? 



Dolomite due to replacement of calcium by magnesium. That the magnesium CarboiiateS 



are due to replacement of magnesium for calcium, and therefore to dolo- 

 mitization, seems to be conclusively shown by the following considerations: 



(1) The first of the various lines of evidence in favor of dolomitization 

 is the composition of the hard parts of sea animals. We do not know any 

 sea animals which deposit more than a small amount of magnesium car- 

 bonate in their hard parts. Forchhammer analyzed a large number of 

 corals and shells of marine animals. In bivalves he found the amount of 

 magnesium to vary from 0.5 to 1 per cent; in cephalopods he found it to 

 be less than 0.5 per cent. In most corals the amount of magnesium is less 

 than 0.5 per cent, although in one species it is 2 per cent and in another 

 species 6 4 per cent." Sharpies analyzed seven species of corals, 6 and does 

 not report magnesium carbonate, although it is to be supposed that a small 

 amount of that compound is present. Forchhammer found in a number of 

 late deposits, which are mainly composed of the remains of animals, that 

 the amount of magnesium is less than 1 per cent. From these results, 

 if it be supposed that formations now dolomitic limestone were originally 

 organic precipitates, it must be concluded either that the magnesium is 

 mainly introduced by a secondary process or that sea animals in early 

 periods used a larger proportion of magnesium for their hard parts than do 

 the present animals. The latter conclusion would be a pure unverified 

 assumption. 



(2) Some of those who hold that dolomites are original deposits, in 

 order to escape the difficulty involved in the supposition that animals have 



" Forchhammer, Georg, Bidrag til Dolomitens-dannelshistorie: Oversigt over det Kongelige Danske 

 Videnskab. Forhandlingar, Copenhagen, 1849, p. 89. See also Bischof, Gustav, Chemical and physical 

 geology, London, 1855, vol. 2, pp. 48-49. 



6 Sharpies, S. P., On some rocks and other dredgings from the Gulf Stream: Am. Jour. Sci., 3d 

 ser., vol. 1, 1871, p. 169. 



'■Forchhammer, cit., p. 89. Also, Bischof, cit. , p. 48. 



