THE BOULDEE VALLEY REGION. 141 



The general trend is approximately X. 1"> W ., based on measurements 

 furnished by the mine superintendents, made in the several cross entries <>t' 

 old workings nol now accessible. A single poinl of observation was possible 

 al the time of the survey, namely, in the Star mine, aboul 900 feet north 

 of the shaft, where then- appears to be a local variation of 8 in trend, to 

 X. "_'.'! W. The inclination of the fault plane w as 30 to the east, or toward 

 the downthrow, and though there was no possibility of estimating the 

 amount of stratigraphic displacement, the superintendent of the mine, 

 from former observations, regarded it as generally not over .'!<> feet. 



The canal fault. — This appears at a single ] i« dm only — in the railroad 

 cut rounding the eastern end of the Lake mesa. It is a fracture apparently 



independent of the general system of faults of the region. The fault plane 

 is vertical; the downthrow north; the opposing beds, the upper and lower 

 layers of sandstone B of the basal series of the Laramie. The stratigraphic 

 throw is, therefore, about oil feet. South of the fault, sandstones 1> and A, 

 with the thin lignite hand between, are crumpled into an anticline, which 

 enters inure or less into the structure of the mesa: north of the fault, the 

 strata, though horizontal at the line of break, within a short distance acquire 

 a northwest dip and form the eastern side of the DavidsOD svncliiie. 



The vaimom fissure. — This is a fracture, apparently without dislocation. 

 Idled with eruptive material. Its discussion falls under the chapter on 

 eruptive geology. 



DISCUSSION OF THE GENERAL PLAINS STRUCTURE. 



The six transverse sections of the Denver field (see PI. IV) show the 



geological structure of the prairies, the structural connection of prairie with 

 foothill and range, and the stratigraphical relations of the formations 

 involved. They will be designated as follows: 

 I. The Boulder section. 

 II. The I )avidson section. 



III. The Ralston section. 



IV. The Green Mountain section. 



V. The Hear ( 'reek section. 

 VI. The l'lllin ( 'reek section. 



