28 THREE GEOLOGICAL PROVINCES. 



sharp ridges are but the remnants left from the cutting away of a plateau 

 like step which once followed along the mountain face. These ridges also 

 end quite similarly along a pretty straight line, and descend to rather a 

 uniform level. Regarding now more particularly the northern ten or fifteen 

 miles of the high range, which includes but four or five of the ridges, it is 

 observed that at the base of each steep end the lowered spur does not con- 

 tinue on as a sharp ridge but slopes off, a flat surfaced, plateau like area, 

 descending gently eastward. Since, upon the corresponding area at the 

 base of the northernmost ridge, great quantities of debris of the Lower 

 Cretaceous sandstones were found, abundantly proving that they covered 

 the area, it appears that all of these flattish areas either are now, or have 

 comparatively recently been, covered with the same sandstones. Such 

 features would seem to indicate that the Cretaceous had once extended high 

 up, or quite over the whole range, and that the latter, in its upfolding, had 

 received the most pronounced uplifts along certain well-defined lines, the 

 intervening portions not being tilted up at high angles. It is by such a 

 process that the front range, at least from the Big Thompson to the South 

 Platte, has received much of its uplift. Major Powell and Mr. Gilbert have 

 noticed similar folds in the Kaibab Plateau and adjacent regions on the 

 great Colorado Plateau of Northern Arizona, though there the sedimentary 

 beds have not (by many a thousand feet) been stripped by erosion from off 

 the underlying rocks. It is a form of mountain building which I think is 

 not uncommon in the West." 



I am inclined to think that the purposes of orology will be better sub- 

 served by classing this structure as a type distinct from that of the Kaibab 

 structure, rather than as a modification of it. The general arching of the 

 strata between the lines of maximum flexure or faulting, allies it some- 

 what to a true anticlinal ; and so far as my studies go these lines of great- 

 est flexure have many more complexities than the faults and monoclinal 

 flexures usually found in the Plateau Province. Hence I have classed it as 

 a distinct type and called it the Uinta structure. 



We already know that the spaces between the broad upheavals, of 

 which the ranges themselves are composed, are complicated by many anti- 

 clinal and synclinal flexures and by many faults, but the whole structure of 



