358 GEOLOGY OF THE DENVEE BASIN. 



several hundred feet northwest of the fracture For about a mile south- 

 west of this the position of the coal may easily be traced across the bottom- 

 lands by reference to the basal sandstones which arc prominently exposed 

 in the bluffs south of the valley. The latter, however, finally sink beneath 

 the bottoms as distance up the valley is gained, and, with the exception of 

 some irregular outcrops opposite the point where the Denver, Marshall 

 and Boulder Railroad crosses Coal Creek, the bluffs for their entire length 

 are here occupied by coal measures or upper Laramie strata. The coal in 

 the upper valley is found at the depth of HiO feet in a shaft a short distance 

 north of the rim of thesyncline and near the point where the Boulder wagon 

 road crosses the creek. It reaches the surface at some point southeast of 

 this, passes into the bluffs, and then southwestward beneath the wash of the 

 prairie until the western rim of the syncline is encountered or until, by 

 the diminished displacement along the Louisville fault in this direction, it 

 has sunk to meet the seam on the opposite side, leaving the surface rim of 

 the syncline again open along here. Southeast of the Louisville fault, 

 in the uplands between this and Rock ('reek, the coal measures probably 

 lie at a considerable depth beneath the surface, forming here also an open 

 lip to the syncline. 



Configuration of the interior of the syncline. A tl'ailSVerse SectioU of tile ( 'oal ( 'reek 



syncline at almost any point would show it to lie of gentle curvature 

 and shallow depth. The greatest distance beneath the surface at which 

 the coal occurs is probably in the region of the Lafayette mines, where 

 tlie top of sandstone B is reached at an average depth of .260 feet. In the 

 eastern mines of the Louisville group this horizon is reached at nearly the 

 same depth, hut west of the railroad a decrease to 170 feet has taken 

 place, which is probably maintained nearly to the southwestern limit of the 

 basin. In the Mitchell group of mines the seam most generally worked — 

 probably that occurring immediately over sandstone B — lies at an average 

 depth of about 100 feet below the surface of the valley bottom. South of 

 the Lafayette-Louisville portion of the syncline, beyond the open rim of 

 the basin, the depth probably increases considerably, there being thence 

 an apparent southeasterly dip for the entire northwest portion of the Denver 

 Held. With the usual thickness of the coal measures, the foregoing depths 



