324 GEOLOGY OF THE DENVER BASIN. 



the beds are horizontal but belong to the upper Laramie, occurring well 

 up in the series of clays, a single locality presenting this condition, the 

 Scranton area, 20 miles northeast of Denver. 



The workable coal beds, whether of the upper or lower division of the 

 Laramie, occur as deposits of irregular outline and distribution from one 

 to several miles in area and of a thickness of from 3 to 14 feet. Their 

 presence or absence, even in localities where the measures have been 

 subjected to combined structural and erosive influences, as along the 

 northwestern edge of the formation, depends upon original conditions of 

 deposition, and in consequence of the uncertainty of these, actual coal- 

 bearing areas at depths beneath the prairies obviously can not lie deter- 

 mined except by exploration with drills. That such areas exist can not 

 be doubted, for their presence along the exposed margin of the coal meas- 

 ures is repeatedly proved, and the sequence of events and the conditions 

 of deposition must have varied but slightly tor any part of the basin. 



Along the western rim of the basin the coal measures suffer no visible 

 interruption by reason of dynamic agencies, though the continuity of the 

 beds is frequently broken by nondeposition. In the northern portion of 

 tin- basin, where the measures, occur at comparatively shallow depths and 

 for a zone of 5 or 6 miles within the periphery of the Laramie are within 

 easy reach of the surface, combined disturbance and erosion have caused 

 great irregularity of occurrence. 



The coal of the lower Laramie belongs eminently to a period of 

 sandstone deposition; that of the upper Laramie to a period of clays with 

 but slight association of sandstone. The heavy sandstones that do occur 

 just above the Scranton seam are not of Cretaceous but of Tertiary age, 

 and lie unconformably upon the coal and other beds of the Laramie. 



Coal occurs at eight or ten horizons within the 200 feet of strata 

 constituting the lower Laramie coal measures, but of these five is the 

 maximum number showing a workable thickness in any one locality in the 

 basin, the usual number being two or three. On account of the covered 

 outcrop and the variability of the measures above the basal sandstones, 

 both in number of seams and character of sedimentation, the identity of a 

 seam can not ordinarily he determined beyond the comparatively limited 



