364 W. T. LEE UNCONFORMITY IN THE SO-CALLED LARAMIE 



bles of petrified wood that may have been derived either from the Dakota 

 or from the Cretaceous coal measures ; red sandstone that could have come 

 only from the red beds; cherty limestone with impressions of crinoid 

 stems; a variety of cherts; quartz; quartzite; jasper; igneous rocks, some 

 of which are coarsely crystalline, others fine-textured, such as are found 

 in the dikes of the mountain region; and fragments of feldspar, most of 

 them completely kaolinized, but some of them retaining their original 

 form perfectly enough to show cleavage faces. The significance of these 

 pebbles in showing the amount of erosion is made evident by reference to 

 the thicknesses of the underlying strata shown in the generalized section 

 previously given. 



Measure of Erosion 



An estimate of the amount of uplift and erosion represented by the un- 

 conformity involves the difficult problem of the distribution of land and 

 sea and the altitude of the Rocky Mountain region during Cretaceous 

 time. The assumption that a land-mass of crystalline rocks persisted in 

 the mountain region throughout the Cretaceous period might satisfac- 

 torily account for the conglomerate without renewed uplift, were it not 

 for the thick underlying bed of fine textured Cretaceous shale; but it is 

 difficult to understand how a coarse conglomerate could be derived from 

 a land-mass that had furnished no coarse material during the accumula- 

 tion of marine shale more than 3,000 feet thick, until that land was re- 

 elevated. If it be conceded that the Rocky Mountain region of Xew Mex- 

 ico was lowlying or submerged during the greater part of the Cretaceous 

 period, and that uplift and renewed erosion preceded the deposition of 

 the post-Laramie conglomerate, it remains to inquire what thickness of 

 sediment was removed in order to expose to erosion the rocks represented 

 by the various pebbles. 



Again, it must be confessed that further investigation is necessary 

 before definite figures can be given, for it is not known how far the for- 

 mations now upturned in the foothills region originally extended west- 

 ward over the present mountain area, nor have the thicknesses of the 

 older sedimentary rocks in the Raton field been measured. The Cre- 

 taceous shale near Raton is known from well borings to be something 

 more than 3,000 feet thick, and it is apparently much thicker than this 

 near the mountains. The Dakota sandstone is about 200 feet and the 

 Morrison shale at least 300 feet thick. The Red beds are faulted and 

 otherwise greatly disturbed in this region, and their thickness can only be 

 estimated at this time, but it is probably not less than 10,000 feet. 



