122 GEOLOGY OF THE DENVER BASIN. 



preclude reliable suggestion as to the depth at which coal measures or 

 water-bearing sandstones may be found. 



structure. — The strata underlying- the Boulder Valley display numerous 

 folds and faults, which probably had a common origin and were approxi- 

 mately contemporaneous with one another. While the results of the once 

 active forces are visible over the whole area, the general effects are not 

 marked, the rolls being gentle, the folds without sharpness, the interfault 

 blocks but slightly tilted — 3° to 20° — and the faults showing stratigraphical 

 displacement rarely above 300 feet. These phenomena are probably die 

 results of compression, the forces of which, from the evidence adduced in 

 the disposition of the rolls and fractures, must have acted along three 

 different lines— E.—W., X. 60° W.— S. 60° E., and N. 30° W.-S. 30° E., 

 these being the directions at right angles to the three systems of trends 

 that distinctly appear in the field. There is much regularity in the occur- 

 rence of both folds and faults; the latter are, with the exception of a few 

 minor cross-fractures, all strike faults; the interfault blocks, where simply 

 tilted and not folded, dip eastward; and the leading synclines closely 

 resemble one another in structure and manner of development. 



THE SYSTEM OF FOLDS. 



The important synclines of the Boulder Valley are: («) The Davidson, 

 King between the main fault of the Marshall subsystem and the Davidson 

 fault, diagonally crossing the mesa of this name and extending beyond 

 southwestward; (b) the Eggleston syncline, at the eastern end of the 

 Lake mesa; (V) the Coal Creek syncline, divisible into five subsynclines, 

 together occupying the valley of both upper and lower Coal < "reek and 

 the broad, low ridge north and west of it. Smaller rolls are numerous, 

 particularly along the western slopes of the divide east of Coal ('reek. 

 The region west and north of the Boulder creeks, underlain by the Mon- 

 tana group, ij'ives little evidence of folds, a general easterly dip prevailing, 

 slightly variable from point to point, but distinctly shallowing as distance 

 from the foothills is gained. 



The Davidson synciinc. — This reaches its greatest development in the Davidson 

 mesa, the broad, sweeping fold showing in both northern and southern 

 faces, its axis, with a S. 23° W. trend, passing across the mesa at the 



