82 GEOLOGY OF THE DENVER BASIN. 



Within the Denver field the folds en echelon are of limited size. They 

 arc tour in number, occurring just north of Ralston, Coal, and South 

 Boulder creeks, and south of Fourmile Creek. 



That north of Coal Creek is in a region of special structural irregu- 

 larity, and will be described in connection therewith. 



The fold at Ralston ( Ireek is the most advanced; it shows complete in 

 the Trias, on the southern face of Ralston Peak, where the axes of both 

 anticline and sycline dip steeply south; in the Dakota the fold is less 

 pronounced, while in the Niobrara the effect is limited to merely a slight 

 divergence from the normal trend. 



The fold north of South Boulder Creek is of the same character as 

 the foregoing, but less developed. The strata are merely offset to the east, 

 with but slight evidence of actual crumpling. In the line of this fold, 

 however, is the prominent fault twinning the South Boulder Peaks, to be 

 described hereafter. 



The northernmost fold in the field is even less developed, and shows 

 in the Dakota only. 



THE REGION ABOUT GOLBEN. 1 



INTRODUCTION. 



The type of geological structure about to be discussed was worked 

 out by the writer in 1884. Upon detailed study it may prove to be of 

 common occurrence along the base of the Rocky Mountains, a recurrence 

 in a less developed form having already been observed in the vicinity of 

 Boulder, a tew miles north of the present area. The type consists in a suc- 

 cession of unconformities appearing one after another at various geological 

 horizons, the explanation of which is found in the forces acting in the 

 general uplift of the Colorado Range, from which have been developed 

 certain secondary forces that have from point to point brought about the 

 elevations upon which the unconformities depend. 



1 The accompanying geological map is an enlarged portion of the general map of the Denver Basin. 

 The profiles are based upon this, and are developed by construction from it and from one another 

 by actual measurements, with the necessary reductions lor thickness. The two sketches on pages 

 !i> and ! i! I illustrative of the manner in which the folding occurred and its relations to that of the main 

 range, are, however, diagrammatic, although based upon the historical facts developed in the text. 



