MEASURE OF EROSION 385 



Assuming that the formations once continued westward with undi- 

 minished thickness over the region now occupied by the southern end of 

 the Eocky mountains, there must have been differential uplift and the 

 removal of at least 3,600 feet of sediment before the pebbles of Dakota 

 sandstone were obtainable, and erosion of at least 4,100 feet before the 

 red sandstone was reached. The pebbles of igneous and metamorphic 

 rocks that constitute the principal part of the post-Laramie conglomerate 

 may have come from the coarser portions of the Eed beds. There were 

 boulders enough in the Eed beds to have supplied the pebbles of the post- 

 Laramie conglomerate, but the granitic material in the Eed beds is re- 

 garded as quantitatively inadequate to furnish the feldspar which forms 

 a considerable part of this conglomerate. It is probable that the granite 

 rocks underlying the Eed beds were exposed to erosion and furnished the 

 feldspars, in which case the differential uplift and subsequent erosion 

 could have been scarcely less than 15,000 feet. This estimate will proba- 

 bly be modified after further investigation, but the statement is amply 

 justified that the unconformity in the Eaton field represents erosion com- 

 parable to the post-Laramie erosion of the Denver region, which Cross 5 

 places at 14,000 feet. 



Correlations 



In some places in the Eaton field the Trinidad sandstone contains 

 marine invertebrates which, according to T. W. Stanton, who has identi- 

 fied them, are of Montana age, belonging to the same fauna as that of the 

 underlying Pierre shale. In other places it contains thin beds of coal 

 and fossil leaves similar to those in the overlying coal-bearing rocks. 

 Undoubtedly the Pierre shale, the Trinidad sandstone, and the coal beds 

 below the unconformity represent practically continuous deposition. The 

 stratigraphic succession is essentially the same as .that in the Denver 

 region, where the Laramie is the last of the conformable Cretaceous series 

 and the marine beds contain invertebrates by which they are correlated 

 with beds of similar position in the Denver section. However, the rocks 

 of the Eaton field that have the stratigraphic position of the Denver 

 Laramie have yielded 15 species of plants, named in the accompanying 

 table, which, according to F. H. Knowlton, who has identified them, are 

 apparently older than Laramie. The species common to the Denver 

 Laramie, as shown in the table, are those known to have a long time range 



6 Whitman Cross : Age of the Arapahoe and Denver formations. U. S. Geological Sur- 

 vey Monograph no. 27, 1896, p. 207. 



XXXII — Bull. Geol. Soc. Am., Vol. 20, 1908 



