196 REPORT UNITED STATES GEOLOGICAL SURVEY. 



sunk. That is, the strata at that particular point have been entirely 

 reversed by the uplifting force of the outflowing trap; so that the shaft 

 was begun in strata of the Fox Hills Group and continued downward 

 until those of the Laramie, including- the coal, were reached in reversed 

 order. 



This bed of coal is believed to be the same that is now worked in the 

 Laramie strata of Table Mountain near Golden City, its change of thick- 

 ness and quality within that distance not being unusual with the coal- 

 beds of the Laramie Group. 



This explanation makes it evident that Gyrena f kolmesi Meek is a 

 Cretaceous instead of, as Meek supposed, a. Tertiary fossil, even without 

 the conclusive testimony of the associated Sca/phite. 



Besides this, by carefully cutting away the embedding shale from an 

 authentic specimen of that species, which I obtained from the same layer 

 that furnished the type specimens, I discovered that it has not the hinge 

 characteristics of Gyrena, but has those of Mactra, or at least of a section 

 of that genus ; thus confirming the marine character of the stratum con- 

 taining it, which was indicated by the associated Scaphite. These facts 

 alone would seem to be sufficiently conclusive, but within the last few 

 months Mr. Lakes has sent to the office of the Survey some specimens of 

 the Gyrena f Jiolmesi of Meek, collected by him from the Fox Hills strata 

 on Bear Creek, near Morrison, about twelve miles southward from the 

 place where that species was originally discovered. The shells sent by 

 Mr. Lakes were imbedded in a small mass of stone, and imbedded in the 

 same mass I also found a fragment of a Scaphite, evidently of the same 

 species as that which was found in the layer that furnished the type 

 specimens of Gyrena? liolmesi, on Balston Creek. It also contained a 

 fragment of a Baculite. These facts are conclusive as to the Cretaceous 

 age of Mr. Lakes's fossils, even without the field-label, which was " Bear 

 Creek, 750 feet below the coal-bed." The coal-bed referred to is that of 

 the Laramie Group and which is not far from its base. 



The questions of interest involved in the discussion of the phenomena 

 observed in connection with the fossils of the coal-shaft on Balston Creek 

 are in part purely geological and in part paleontological, and some of 

 them are so important that the subject has here received more attention 

 than would otherwise have been given to it. 



The location of the great trap vent upon the line of flexure of the 

 strata, which are upturned against the flank of the Bocky Mountains, 

 would seem to indicate that the outburst came there because of the 

 weakening of the strata by having been flexed, thus lessening the re- 

 sistance to upward pressure. But while the strata are continuously 

 flexed along the base of the mountains, the vent is circumscribed, and is 

 not in the form of a dike, nor a lengthened sheet along the line of flex- 

 ure. Besides this, the dike at Valmont, fifteen miles to the northward, 

 is not upon, but several miles eastward of, the line of flexure, and at right 

 angles to it. It is doubtless true that the inclined posture of the strata, 

 when the trap burst through, facilitated the overturning of those that 

 were upon the outer side of the vent; those between the vent and the 

 granite mass of the mountains offering greater resistance to the pressure 

 were comparatively undisturbed. 



It is also desirable to elucidate every question which bears upon the 

 order of succession of the brackish- and fresh- water deposits upon those 

 of marine origin, and the consequent order of succession of invertebrate 

 types. Coal-making conditions are known to have existed at times in 

 both the Colorado and Fox Hills Cretaceous epochs, in what are now 

 the regions .west of the Bocky Mountains, but they are believed not to 



