EVIDENCES OP THE UNCONFORMITY 363 



Sections were measured at short intervals in the escarpment along the 

 eastern margin of the field, and these have been plotted to scale in figure 

 C, plate 28, in order to show the irregularities in the line of unconformity. 

 In figure B of the same plate measured sections are arranged in order 

 from west to east, showing the thinning of the Cretaceous coal measures 

 away from the mountains. The difference in thickness may be due in 

 some measure to original deposition, but more probably is due to the par- 

 tial removal by erosion of the Cretaceous beds. These facts apparently 

 force the conclusion that no highlands existed near the Eaton field which 

 could by any known process, except that of invigorated erosion due to 

 mountain uplift, furnish the pebbles found in the conglomerate at the 

 base of the post-Laramie formation. There can be no reasonable doubt 

 that erosion preceded the formation of the conglomerate, and the thinning 

 of the Cretaceous coal measures from west eastward is best explained as 

 due to this erosion. However, the most convincing proof of the duration 

 of the erosion interval is found in the composition of the conglomerate. 



Conglomerate 



The base of the post-Laramie formation in nearly all parts of the Eaton 

 field is conglomeratic. In the western part, where the formations are 

 upturned along the base of the mountains, and in Vermejo park, the con- 

 glomerate is coarse, massive, and resistant and forms a prominent hog- 

 back (see plate 29). It is coarsest at the base, where through a thickness 

 of 100 feet the pebbles attain a maximum diameter of five inches. Above 

 this massive basal part there is a small amount of carbonaceous shale with 

 thin seams of coal, but for 600 feet above the unconformity the sediments 

 are principally coarse grained sandstones locally conglomeratic. 



Eastward, or away from the mountains, the basal portion of the con- 

 glomerate thins and the pebbles are smaller. In the eastern part of the 

 field the conglomerate is well developed as far north as Eed river. Near 

 Eaton it is doubtfully represented by a quartzose sandstone, but still 

 farther toward the east the conglomeratic character reappears, as is shown 

 graphically in figure B, plate 28 ; also the upper part of the conglomerate, 

 as represented in the foothills at the eastern edge of the field, becomes 

 finer textured toward the east, loses its conglomeratic character, and ap- 

 parently is represented in the eastern part of the field by the cliff-making 

 sandstones that occur in the lower 400 feet of the formation east of Eaton. 



In the conglomerate were found pebbles of coal; sandstone similar to 

 the Trinidad ; quartzose sandstone similar to the Dakota ; pebbles of con- 

 glomerate similar to the conglomerates of the Dakota ; well rounded peb- 



