614 E. BLACKWELDER ORIGIN OF BIGHORN DOLOMITE OF WYOMING 



The question as to depth of water is less readily answered. The preva- 

 lence of corals suggests shallow or at least moderate depths, but the evi- 

 dence loses something of its cogency when we consider that the predomi- 

 nant corals are of the solitary types, or such compound forms as grow in 

 small isolated colonies rather than in reefs. If, as suggested in later 

 pages, the peculiar structures of the Bighorn dolomite have been built by 

 marine algae of the bank-forming habit, then we have some reason to 

 think that the depth of the sea was less than 120 meters. The modern 

 colonial algae have a greater batkymetric range than reef corals, but live 

 only within the zone of effective light transmission. The occasional beds 

 of cross-laminated calcarenyte seem to show plainly the action of cur- 

 rents strong enough to move and assort sandlike material. This also 

 indicates a depth of less than 100 meters. Taken together these con- 

 siderations seem to justify the inference that the Bighorn dolomite was 

 deposited in continental rather than abysmal waters. 



The conditions of temperature and salinity and the character of the 

 marine solution can hardly be discussed with profit under present circum- 

 stances, although they were doubtless of the highest importance in deter- 

 mining the character of the formation. 



ORIGIN OF THE STRUCTURES 



The ill-defined but none the less existent branching structures with 

 which most layers of the Bighorn dolomite are filled, and which express 

 themselves on freshly broken surfaces as vague color-mottling and on 

 weathered outcrops as alternating pits and ridges, have never been satis- 

 factorily explained. Weed 11 notes that the darker portions owe their 

 color and fetid odor to nitrogenous matter, but makes no further sugges- 

 tion. Darton 12 ascribes the peculiarities of the weathered surface to a 

 network of siliceous matter imbedded in the rock. To quote : 



"The massive limestone which constitutes the greater part of the formation 

 is a rock usually of light buff color, somewhat darker when weathered, filled 

 with a coarse mat or network of irregular, siliceous masses, mostly from one- 

 half to one inch in diameter. On weathering, this siliceous material stands 

 out a half inch or more on the rock surface as a ragged network, the purer 

 rock between having been dissolved. . . . This feature and the very mas- 

 sive bedding are characteristic." 



The origin of the supposed siliceous network is not discussed. As a whole 

 ion's explanation seems to be untenable, because the chemical analyses 



*W. II. Weed: Geology of the Little Belt Mountains, Montana. U. S. Geological 

 Survey, 20th Ann. Kept, pt. ill. 1900, pp. 271-461. ^eoiogicai 



« n N n H ', ^ art0 ° : FIsh remains In Ordovician rocks In Bighorn Mountains, Wyoming. 

 Bull. Geol. Soc. Am., vol. 17, 1906, pp. 541-566. 



