DOLOMITIC SEDIMENTS. 279 



generally the granular structure characteristic of dolomites, and under the 

 microscoj^e it is seen that there is little or none of the twin structure pecul- 

 iar to calcite, and that they are therefore composed, not of a mixture of 

 calcite and carbonate of magnesia, but of true dolomite or double carbon- 

 ate of lime and magnesia. The upper bed of the Robinson limestone, on 

 the other hand, and also the few limestones of the Upper Coal Measure 

 formation that were examined are true limestones and have a characteris- 

 tically different appearance from the dolomites in the hand specimen. They 

 are fine grained and compact, instead of granular, generally of light color, 

 and often have the conchoidal fracture and fine texture of a lithographic 

 stone. The lime and magnesia contents of twenty different specimens 

 of limestones from different horizons and localities are given in Table VI, 

 Appendix B. 



It is also noteworthy that all of these limestones, as far as tested, were 

 found to contain chlorine in appreciable amount. Microscopical examination 

 of the Blue Limestone collected at Leadville, whose contents in chlorine 

 amounted to one-tenth of one per cent., showed that it probably occurs in 

 the form of a solution of chloride of sodium, in extremely minute fluid 

 inclusions within the grains. 



These investigations were made in the hope that they might throw 

 some light upon the cause and manner of formation of dolomites in gen- 

 eral. It can only be said that it seems evident that the magnesia is an 

 original constituent of the rocks, and not introduced later by metamorphic 

 action. It were difficult to conceive of such an action, for instance, in the 

 case of the Robinson Limestone, the upper fifteen to twenty feet of which 

 are almost chemically pure carbonate of lime, while the lower ten feet con- 

 tain less than 88 per cent, of carbonate of lime, the rest being carbonate of 

 magnesia and insoluble material ;, or how such metamorphic action should 

 be so widespread and uniform over this great area and yet stop at a given 

 bed or horizon. 



T. Sterry Hunt,^ who, with others, has advocated the theory that dolo- 

 mites are formed by the actual precipitation of the carbonate of magnesia, 

 maintains that its separation requires the absence of chloride of calcium 



' Chemical and Geological Essays, Bostou, 1875, j). 92. 



