LOCATION AND CONDITION OF THE ROCKS DESCRIBED 359 



northern, or Colorado, part of which is well known through the writings 

 of Hills. 3 The Eaton field extends from the Colorado line southward 

 beyond the Cimarron river, a distance of about 40 miles, and from the 

 base of the Eocky mountains eastward about 50 miles. Near Eaton the 

 rocks outcrop in the steep slopes of the lava-capped mesas that are fre- 

 quently, though erroneously, called the Eaton mountains. To the south 

 and west of Eaton the rocks have been deeply eroded and good exposures 

 are numerous; also in the western part of the field the rocks are well 

 exposed where they are upturned in Vermejo park and along the base of 

 the Eocky mountains for a distance of about 5 miles south of the Colorado 

 line. Farther south the outcrop is obscured by slide rock and the rela- 

 tions complicated by intrusions of igneous rock. From Ute park east- 

 ward in the Cimarron canyon and northward along the southeastern 

 margin of the coal field many well exposed sections of the rocks were 

 measured. Illustrations of these exposures are given in plate 30. 



Eock Formations 



Although little can be said of the older rock formations of the Eaton 

 region, some knowledge of their general character and thickness is neces- 

 sary in order to appreciate certain facts relating to the unconformity here 

 described. 



The oldest formations of the region consist of the ancient crystalline 

 and metamorphic rocks, probably of pre-Cambrian age. These are over- 

 lain in some places by sediments of Pennsylvanian age and in other places 

 by the red beds of the eastern Eocky mountains that probably range from 

 late Pennsylvanian to Triassic. The red beds are here coarsely conglom- 

 eratic, containing boulders of crystalline and metamorphic rocks derived 

 from the older complex, and are so faulted and otherwise disturbed that 

 it is difficult to measure their thickness, but 10,000 feet is believed to be 

 a conservative estimate. 



The red beds are overlain by about 300 feet of Morrison shale and 200 

 or more feet of Dakota sandstone. Above the Dakota is a thickness of 

 3,000 feet or more of marine Cretaceous shale, the upper part of which is 

 referred on paleontologic evidence to the Pierre. This shale grades up- 

 ward through a transition zone of sandy shale, which Hills calls lower 

 Trinidad, 4 to the massive Trinidad sandstone, which varies from 50 to 120 

 feet in thickness and which contains a marine fauna allied to that of the 

 Pierre shale, but which in some places contains also thin beds of coal and 



3 R. C. Hills : U. S. Geological Survey, Elmoro folio, no. 58, 1899 ; also U. S. Geologi- 

 cal Survey, Spanish Peaks folio, no. 71, 1901. 



* R. C. Hills : U. S. Geological Survey, Elmoro folio, no. 58, 1899. 



