GENERAL CHARACTERISTICS 613 



meter thick. In these coarser-grained portions there are also roundish 

 bodies and others of more or less irregular shape, most of which consist 

 of a single shapeless crystal of dolomite of abnormal size. They were 

 probably segments of crinoid or cystid columns and bits of shells; but 

 very few are now distinctly recognizable, because their outlines and inter- 

 nal structures have been obliterated by recrystallization. It is significant 

 that these bits of fossils occur only in the coarse-grained matrix and 

 not in the dark bodies. 



That the Bighorn dolomite contains fossils is well known, for traces of 

 them may be observed on the weathered outcrops of almost any of the 

 beds, although almost never well preserved and generally scarce. The 

 commoner kinds are such corals as Streptelasma, Holy sites, and Favo- 

 sites; crinoid or cystid segments and unrecognizable brachiopods. These 

 fossils are very rarely visible on freshly broken surfaces, and they appear 

 on weathered faces only by virtue of the fact that they consist of large 

 crystals which are less rapidly dissolved than the finer grains. 



At both top and bottom the Bighorn is generally limited by discon- 

 formities — irregular cavernous contacts along which the dolomite is ab- 

 ruptly set off from the adjacent formations. In some exposures these 

 contacts are marked by fragmental material apparently derived from the 

 subjacent terrane and imbedded in a dolomite matrix. Generally the 

 fragments are not distinctly water-worn and there is a conspicuous ab- 

 sence of quartz sand. At various horizons within the formation thin 

 cross-bedded layers of cream-colored calcarenyte 9 occur, now somewhat 

 altered by crystallization. These beds, evidently affected by currents, are 

 noteworthy because they resemble the porous cream-colored matrix of 

 many of the more massive beds. Even in the calcarenyte the microscope 

 shows no foreign matter whatever. 



Interpretation 



general significance 



It is doubtless unnecessary to argue that the Bighorn dolomite is ma- 

 rine. If any specific evidence were required, the wide distribution of 

 corals throughout its mass would be sufficient. 10 Thai il also represents 

 deposition far from land is strongly suggested by the absence of recog- 

 nizable clastic debris. It could hardly be purer if il had been deposited 

 in mid-ocean. 



°A rock consisting of sandllke calcareous particles cemented (Grabau). 



10 Weller has recently called attention to the fuel thai the fossils In most dolomite*, 

 such as the Niagara, Galena, and Knox of the Eastern States, are essentially like those 

 of ordinary marine limestones. (Bull. Geol. Soc. Am., vol. 22, 1911, p. 881.) 



