WHY THE ROCK IS A DOLOMITE 619 



convincing. At least five possible hypotheses may be suggested as worthy 

 of consideration in reference to the Bighorn: (a) erosion of preexisting 

 dolomite rock ; ( b ) direct chemical precipitation of the mineral dolomite ; 

 (c) formation directly by organisms secreting dolomitic shells; (d) depo- 

 sition, either organically or chemically, as lime carbonate, but progress- 

 ively altered to dolomite by magnesian solutions while yet unburied; (e) 

 dolomitization of consolidated limestone long after deposition by circulat- 

 ing underground waters. It must also be recognized as a possibility that 

 the true origin of the Bighorn formation involved a more or less complex 

 combination of two or more of these processes. 



The first suggestion may be dismissed with scant attention. Although 

 the few layers of calcarenyte are made of fragmental material and show 

 the action of currents, the prevailing beds of the formation do not. Still 

 more convincing is the purity of the dolomite. If of fragmental origin, 

 it would have to be derived from land-masses composed entirely of dolo- 

 mite ; but for so widespread and thick a terrane this seems incredible. 



There seems to be but little reason to think that the Bighorn dolomite 

 has been produced by the direct and initial precipitation of the double 

 carbonate through the partial evaporation of sea-water, although some 

 dolomites may have originated in that manner. The whole, terrane is 

 evidently marine in origin, and it entirely lacks the characteristics of 

 calcareous deposits produced by the desiccation of inclosed seas. It seems 

 to be true that dolomite has not been found in course of precipitation on 

 any part of the present sea-bottom, nor have chemists thus far devised, so 

 far as I have learned, any laboratory conditions competent to cause the 

 precipitation of dolomite and at the same time likely to be realized on a 

 large scale in oceanic waters. These considerations lead me to seek some 

 other mode of formation as more probable. 



Under the third hypothesis it may be supposed that certain early Paleo- 

 zoic organisms normally constituted their shells of dolomite rather than 

 of lime carbonate, and that the Bighorn rock originally consisted of such 

 remains. Since no organisms are now known to secrete dolomite, this sug- 

 gestion may appear chimerical to the reader. But it must be admitted 

 that many calcareous creatures of the sea now contain small amounts of 

 magnesium carbonate : corals, 0.1 to 11.0 per cent and coralline alga?, 

 1.95 to 13.19 per cent. 17 It is conceivable, although perhaps unlikely, 

 that in early geologic periods organisms may have used a much larger pro- 

 portion of magnesium in their shells. At times of greatly diminished 

 lands, as in the Middle Ordovician, the terrigenous supply of lime salts 



1T P. W. Clarke: Data of Geochemistry. U. S. Geological Survey, Bull. 491, 1911, p. 

 540. 



