coal. 325 



area in which it is specially developed, The basal sandstones, A and B, 

 and the bed of oysters, about 12 feel above their summit, are the chief 

 horizons of reference in the scries. 



Notwithstanding the indefinite position of the coal seams in the lower 

 Laramie, their occurrence for the basin in general points to certain horizons 

 at which a workable thickness seems to he mmv frequently developed than 

 at others. These horizons are: Between sandstones A and B, immediately 

 above sandstone l'>, just below sandstone C, and at one or two points 

 between the two last mentioned. Those most commonly developed lie in 

 the shaly beds between the I> and C sandstones. 



The productive area of coal depends upon the thickness of the seam 

 and the purity of the coal. Either of these varies independently of the 

 other. In thickness the beds occur from a knife-edge to 14 feet without 

 parting. In quality the coal may pass from the state of the highest purity 

 for the field to one in which there is a large proportion of earthy matter, 

 to a carbonaceous shale, or even to a shale wholly argillaceous. A 

 combination of the two methods of variation is of frequent occurrence. 



Partings of a more or le>s carbonaceous clay or quartzose sand-rock, 

 varying from a line to a foot or two in thickness, are frequently present 

 and may extend through an entire district. Nearly all carry plant remains — 

 leaves, bark, etc. 



COAL AREAS. 

 INTRODUCTION. 



The coal fields of the Denver Basin are the Foothill, Marshall, 

 Davidson, Louisville, Lafayette, Baker, Erie (embracing the Canfield and 

 Mitchell), "White Rock, and Scranton. Beyond the basin as mapped are 

 the McKissic and Platteville banks, several miles to the north, and the 

 Douglas or Lehigh bank, 6 miles southeast of Platte Canyon. 



With the exception of the Scranton the areas are all geologically 

 related, but have become distinct from one another through faulting, 

 folding, and erosion. The mines of the Foothill region lie along the 

 upturned portions of the measures. The Marshall district adjoins the 

 northern end of the foothill area, its mines confined to the region of gentle 

 dip immediately east of the latter. The Louisville, Lafayette, and Baker 



