382 



GEOLOGY OF THE DENVEK BASIN. 



them from either the earlier or the later set of analyses. The same method 

 of comparison will for like reasons also hold for the foothill coals, inter se. 



Fixed carbon. — Employing either the old or the new suite of analyses, the 

 prairie coals of the lower Laramie closely approximate one another in the 

 percentage of fixed carbon, those of the New Boulder Valley mine alone 

 materially falling below the others. The foothill coals — excluding that 

 from below the 600-foot level in the Old White Ash mine at Golden, 

 which in its percentage of fixed carbon distinctly approaches the coal of 

 the prairie regions — show amOng themselves a somewhat greater range in 

 this constituent than is recognized in the prairie coals, and, in direct 

 comparison with the latter coals, present a remarkable decrease in the fixed 

 carbon. Such alteration as this last is readily explained by the very 

 considerable depths at which weathering of vertical coal beds may proceed 

 and by the easy infiltration, either mechanically or in solution, of foreign 

 material into the interstices and joints of the seams; and a further factor in 

 the decrease may be the actual breaking up of some of the more stable 

 carbon compounds to supply the place of the more volatile hydrocarbons 

 that are possibly given off in weathering. 



A steady increase, however, in the fixed carbon of the coal of the 

 vertical beds takes place as depth is gained. It is (dearly perceptible in 

 the mines about Grolden, where careful sampling afforded the following 

 sequence of percentages: 



voiatiie-combustibic matter. — The average percentages of the volatile hydrocar- 

 bons in the prairie and foothill coals of the lower Laramie, as shown in 

 the older suite of analyses, are, respectively, 33.64 and 34.27; in the 

 Scranton or upper Laramie coal. Ml. 75. 



