c24 e. blackw'elder origin of bighorn dolomite of wyoming 



Summary 



(1) The Bighorn dolomite is widely distributed in Wyoming, and will 

 probably be fonnd to extend through the northern Kocky Mountains from 

 the Black Hills southwestward to Utah and northwestward to southern 

 Montana ; but it is absent in southeastern Wyoming. 



(2) It is of Ordovician age and probably includes Silurian also. 



(3) Chemically it is a very pure, normal dolomite, almost devoid of 

 terrigenous material. 



(4) Fossils are rare and seldom well enough preserved to be identified. 

 The most abundant are corals and crinoid stems. 



(5) Its characteristic coarsely pitted and fretted surface is due to the 

 differential weathering, not of siliceous and calcareous matter intermin- 

 gled, but of compact fine-grained dolomite structures imbedded in a 

 matrix of more coarsely crystalline and more porous dolomite. 



(6) The ill-defined branching structures which are directly related to 

 the pattern of the weathered surface are probably of organic origin. It 

 seems more likely that they represent banks of calcareous algae than any 

 of the plant-like animals. 



( 7 ) The obliteration of the original organic structures is assigned to 

 the process of crystallization of the dolomite. 



(8) In origin the mass of the rock is not in large measure fragmental, 

 was probably not directly precipitated as dolomite or as dolomitic shells, 

 and probably has not been produced by the substitution of magnesium for 

 calcium in a solid limestone. It is suggested as most probable that it has 

 been produced by the almost simultaneous interaction of the bottom layer 

 of sea-water, charged with magnesium salts, on shells, skeletons, and other 

 particles of calcite and aragonite accumulating as loose sediments on the 

 sea-floor. 



(0) The deposits were probably made in an epicontinental sea less 

 than 100 to 200 meters in depth ; and there are considerations which sug- 

 gest that the water was warm, somewhat (but not much) above the aver- 

 age in salinity, and contained more than the usual amount of magnesium 

 salts. 



In conclusion, it gives me pleasure to acknowledge valuable sugges- 

 tions on the reef -making coralline algae of modern seas from Dr. Marshall 

 A. Howe, of the New York Botanical Garden, and from Dr. C. I. Lewis, 

 of the University of Wisconsin, on various botanical questions concerning 

 the algae. Doctor Howe also kindly loaned the photographs of Lithophyl 

 lum antillamm which are used here as illustrations. 



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