366 GEOLOGY OF TI1E DENVER BASIN. 



fault cuts the field from the productive area of the linker mine and the 

 coal lying to the east-northeast. 



The axis of the Lafayette trough lies a little south of the Simpson- 

 Spencer shafts; the drainage being from the mines north and south to the 

 Simpson-Spencer. 'The hollow of the trough is somewhat corrugated with 

 minor flexures of gentle rise, hut its general shape is broad and rather Hat, 

 the dip steepening only at the limits of the area now opened, on the 

 northwest, very gradually. The mines show several faults with various 

 inclinations for their planes, some of which arc at a very acute angle with 

 the planes of stratification. The throws rarely exceed 8 or 1(1 feet. 



The coal of the Lafayette field is of the general character of the Erie 

 coals. It is jointed and works in large blocks. 



A woody structure of the coal is frequently encountered, as though 

 carbonization had taken place in solid blocks, fiber for fiber. Silicified 

 trunks of trees, knots, and branches are here and there found, but so far 

 as observed they lie in no definite direction. 



The mines are all opened by shafts. Sections of the two seams at 

 several points in the district are shown in Figs. A to E, inclusive, PI. XX. 



MITCHELL DISTRICT. 



The productive portion of the Mitchell area is defined on the west by 

 The crest of the ridge separating Coal Creek and Sand Gulch, on the cast 

 by the sharp rise of the beds to the surface along the Coal < 'reek fault, and 

 on the north by the Erie fault, against which the coal measures probably 

 abut with little bending. To the south the held is apparently continuous 

 with the Lafayette. The limits thus defined form the exterior periphery of 

 the field. It is hardly possible that the coal seams should hold workable 

 within the entire area, since their tendency to a rapid variation in thickness 

 is well established. Their general relations arc shown in the sections of 

 shafts, and their structure in the plate of individual beds. 



The coal of the area is bright, hard, and dicy. Both laminated and 

 homogeneous varieties occur. "Mother of coal," pyrite, resin, and white 

 sulphate of lime are present in variable but never high percentages. 



The mines are all worked from shafts. Figs. E to J, PI. XIX, show 



the general character of the seams. 



