coal. 359 



-will permit a general cover of from 50 to 200 feet of upper Laramie along 

 the axis of the syncline, according to locality. East of the axis, along lower 

 Coal Creet, the rover of upper Laramie rapidly diminishes, owing to tin- 

 sharp rise of the strata toward the Coal Creek fault; west of the axis the 

 decrease in depth of this series is more gradual, especially for the northern 

 half of the syncline, where the beds have a very gentle southeast dip, but 

 slightly greater than the slope of the hills. Regularity of structure and 

 depth of the various measures have also been affected by the Louisville, 

 Erie, Jackson-Star, and other, minor faults. 



The structure of the extreme northern part of the syncline, from Erie 

 north, is somewhat obscure, but the general depth of the coal measures may 

 be locally estimated from outcrops in Coal Creek and from mines already 

 opened. In the Boulder Valley and Erie mines the upper seam of coal 

 developed is reported at 70 feet beneath the surface, and a second seam at 

 a depth of 133 feet. Northward the cover of the coal decreases, both on 

 account of erosion and because of a gradual rise of the strata. In the Star 

 mine, near the northwestern rim of the syncline, this rise is 2 feet in 100. 



Faults and rolls of roof and floor of minor importance occur in several 

 mines of the Coal ('reek syncline, but they are wholly local. They will 

 be mentioned hi the detailed description of the mines. 



STRATIGRAPHY AXJ) CORRELATION. 



introductory. — The strata involved in the Coal ('reek syncline and exposed 

 at various points within its confines include the sandstone cap of the Fox 

 Hills, the basal sandstones and coal measures of the lower Laramie, 

 and the lower third of the upper Laramie. All maintain their general 

 lithological features, but the coal measures from point to point present 

 important variations in the width of the individual beds composing them. 

 This variability is particularly well illustrated in the actual union in the 

 Lafayette-Louisville region of two beds of coal which in the Erie held are 

 distinct and separated by a series of sandstones and shales of an average 

 thickness of '_'."> feet, and their point of divergence may even he observed 

 in the former area, the rate being about 1 foot in 50 or 60 until the general 

 width between them is attained, several hundred feet to the north. 



Recognition of the several coal beds and their relations to one another 



