116 ON GORAL REEFS AND ISLANDS. 



ate of magnesia, 1-38, alumina, 0-24, phosphoric acid and silica, 

 a trace. 



We cannot account for this supply of magnesia except by re- 

 ferring to the magnesian salts of the ocean. It is an instance of 

 dolomization, during the consolidation of the rock beneath sea- 

 water, and throws light on this much vexed question. 



This subject is illustrated, and the view we sustain confirmed, 

 by an article on the formation of dolomite from carbonate of lime, 

 published in the Naturwissenschaftliche Abhandlungen edited by 

 W> Haidinger, (4to. Vienna, 1847.) According to von Morlot, in 

 this paper, Haidinger has recently shown both by the frequent 

 association of gypsum with dolomite, and by chemical experiment, 

 that carbonate of lime and sulphate of magnesia, when together, 

 undergo a double decomposition, the magnesia taking the place of 

 part of the lime, and the excluded lime combining with the sul- 

 phuric acid set free. The result is magnesian carbonate of lime, 

 (dolomite,) and hydrous sulphate of lime, (gypsum,) the latter 

 being separated, and either continuing in solution or solidifying, 

 according to the amount formed or the proportion in the water. 

 Yon Morlot gives figures of specimens from different localities in 

 which gypsum and dolomite are intimately associated j and among 

 them are some of fossil corals. 



According to Haidinger, however, some heat is required for 

 this result. Yet in the case of the coral rock and the compact 

 magnesian limestones of our Western States, there is no evidence 

 of the action of such heat ; the subject therefore requires farther 

 investigation.^ 



The circumstance of a chemical change going on between the 

 carbonate of lime and magnesian salt, (for such a change, under 

 some circumstances, must have taken place,) is especially favora- 

 ble for consolidation. When the coral is a fine mud, and the grains 

 are therefore extremely fine, the dolomisation might extend to the 

 grains themselves, as well as the infiltrating material acting as a 

 cement. But when the grains of coral are large, or there are peb- 

 bles, the infiltrated material that might be magnesian would con- 

 stitute but a small part of the whole bed. Hence it is obvious 

 that such formations in cold waters should not always in the 

 mass have the proportions of a true dolomite, (54-2 of carbonate 

 of lime, to 45*8 of carbonate of magnesia ;) they would probably 

 attain such proportions under an ocean during that action of heat 

 required alike for crystallization and chemical combination.* 



* Prof. Horsford, in a paper oh the consolidation of coral reef-rock, read before 

 the last meeting of the American Association at Albany, (August, 1851), attributes 

 this consolidation to the presence of organic matter which undergoes decomposition, 

 as follows : — the sulphur present produces sulphuretted hydrogen ; this changes to 

 sulphuric acid, whence results sulphate of lime and a soluble carbonate of lime; 

 then ammonia (resulting from the nitrogen) carries off the sulphuric acid as sulphate 



