and its Bearing on the Coral Reef Problem. 189 



nating with calcite in certain stalagmitic or agate-like deposits 

 lining cavities in certain parts of the reef limestones. 



AVe may conclude then that our present knowledge suggests 

 that direct chemical precipitation of dolomite in nature, while 

 known, is of restricted occurrence, and of limited geological 

 significance. 



The Clastic Theory of Dolomite Formation. 



Lesler, Phillipi, and Grabau, among others, have suggested 

 that certain dolomites represent either mechanical sediments 

 derived from an older dolomite or from the residuum of lime- 

 stones low in magnesia. No claim, however, is put forward 

 that this method of formation is in any way widespread and 

 the first hypothesis necessarily presupposes the existence of 

 older dolomites from which later clastic deposits of similar 

 composition could be formed. 



The formation of dolomite by differential leaching of slightly 

 magnesian limestones in carbonated water. 



This hypothesis is based on the observation that in slightly 

 magnesian limestones water containing C0 2 will dissolve cal- 

 cium carbonate while magnesium carbonate is only slightly 

 affected. The solubility of calcium carbonate in sea water is 

 very small and has been variously estimated at from 1 in 10,000 

 to 1 in 136,000. In sea water containing 0O 2 in solution the 

 solubility, no doubt, is much greater. Bischoff's and Hard- 

 man's experiments show that, when powdered slightly, magne- 

 sian limestone is attacked by water saturated with C0 2 ; 

 at atmospheric pressure the calcium carbonate is readily 

 dissolved, and very little of the magnesium carbonate goes into 

 solution. 



This process of leaching of slightly magnesian limestones 

 may be pictured as occurring either under subaerial or under 

 submarine conditions. 



Grandjean* in 1844 appears to have been the first to suggest 

 subaerial leaching as a method for the formation of dolomite, 

 and utilized this theory to explain the production of the dolo- 

 mites of the Lahn district. Hardmaiyf- following Bischoff, 

 later accepted this process after experimental tests, to explain 

 the origin of the dolomites of the Carboniferous limestone of 

 Ireland. Hall and Sardesen^: adopted the same view with 

 regard to the origin of the Lower Magnesian series of the 

 Upper Mississippi Valley. Hagboni,§ however, regards surface 



* Grandjean, Neues Jahrb. , 1844, p. 543. 

 + Hardman, Pioc. R. Irish Acad. Science (2). ii, p. 705. 1877. 

 {Hall and Sardesen, Bull. Geol. Soe. Amer., vi, 167, 1895. 

 SHagboni, Nenes Jahrb., 1894, i, p. 262. 



