328 GEOLOGY OF THE DENVER BASIN. 



never important, varied from pits with an annual output of a few hundred 

 tons to nicrc prospects. The history of these mines and the extent to which 

 they have been worked are, in a measure, indicative of the conditions of 

 the beds in their respective regions, but abandonment of mines once well 

 established does not signify that they have been worked out, hut rather 

 that they can no longer produce economically in competition with the 

 mines of the prairies, where the coal is horizontal and within a compara- 

 tively short distance of the surface. 



For much of the distance along the foothills the coal measures are 

 barren, and the extent of the productive portion can be determined only 

 bv most careful prospecting; and it must always be home in mind that, 

 through the irregularity of outline of the original deposit, a bed continuous 

 in outcrop fur a considerable distance may extend to but slight depth, or 

 vice versa. Thus far, continuity in depth has been found to exceed the 

 limits of economic working. 



STRIKES AND DIPS. 



The strike of the coal measures in the foothill region is, with the 

 exception of the portion included within the area of the unconformity 

 about Golden, parallel with the range (N. 15° to 18° W.) in the southern 

 portion of the field, and north or a little west of north in the northern. 

 between Bear and Coal creeks, however, tlie coal measures lie in a broad, 

 westward-sweeping curve, without crumple or fracture of importance. 



The dip of the beds varies from 10° to 15° on either side of vertical. 

 The depth at which the dip of the overturned beds becomes vertical and 

 then normal (easterly) varies from point to point, but in the lower levels 

 of the White Ash mine at Golden between 700 and Sol) feet would consti- 

 tute approximately their vertical portion. Below this the eastward dip may 

 generally be expected, and at 1,200 or 1,400 feet the shallow dip charac- 

 teristic of the prairie occurs. 



The curve from overturn to normal is of such a long radius that 

 probably but slight fracturing of the coal seams has resulted. The seams 

 of workable thickness along the foothills are apparently wholly confined 

 to the series of strata between sandstones 1! and 0, that between A and 13 

 being nowhere with certainty recognized as important. 



