„.„..,v.i GEOLOGY ROCKS EAST OF FRONT RANGE TRIAS. 95 



MARA LMC.J 



exceptional cases, harder beds may rise in ridges of abnormal height, 

 or all may be leveled flat ; the rocky fold may be more gentle or more 

 abrupt, or complex; or the presence of volcanic products may add new 

 features to the scene. Still the normal structure of the region remains 

 the same. 



This region furnishes, then, the key to a knowledge of the rocks which 

 underlie the plains, and, by selecting favorable localities for exposures, 

 we may, in passing from the mountains eastward, pass in succession 

 across the edges of the beds from the lowest to the uppermost. It 

 would thus be found that a thickness of probably over 7,000 feet of 

 sedimentary beds has partaken of the folding. These beds would seem 

 to divide themselves naturally into four groups, which, so far as the 

 preseut burden of proof goes, are referable to tlfe following geological 

 formations, commencing with the oldest: Triassic, Jurassic, Cretaceous, 

 and Eocene(?). The upper part of this series, from the Middle Cretaceous 

 up, being of comparatively softer beds, has been generally eroded back 

 *so far from the mountains, and to such a level and grass-covered sur- 

 face, that exposures are few and scattered, requiring for study more 

 time than could be given to them during the last summer. They have 

 received special study from others, however, so that some general 

 results can be given in the sequel. For the present we will confine our 

 attention to the lower portions of these beds, the Triassic, Jurassic, and 

 part of the Cretaceous. 



In describing these beds it will be found convenient to regard, first, 

 their more detailed structural features, as lithological characters, age, 

 &c. ; and, second, their more general structural features, or the manner 

 of their folding. To illustrate the first class of facts Plate I has been 

 prepared ; and for the second, Plate II, (see end of chapter,) though in 

 dealing with either class it will occasionally be found desirable to refer 

 to both plates at once. 



On Plate I are gathered sections taken at six points along the zone 

 of hog-backs, namely, commencing at the northern one, at Little Thomp- 

 son Creek, Bear Caiion, Ealston Creek, Golden City, (Clear Creek,) Bear 

 Creek, and at the South Platte. The thicknesses of the beds were esti- 

 mated, and are, therefore, only approximate, but checks of various kinds 

 were used to prevent any gross errors, and when the same series were 

 observed by different persons the results were generally very concord- 

 ant. The beds whose thicknesses were thus obtained have their limiting 

 lines carried through to the vertical line forming the left-hand border 

 of each section, upon which, also, spaces of 100 feet each are indicated, 

 while beds expressing general characters of the series, but not so- ex- 

 actly placed, are carried through only to the line next to the left edge 

 of the section. The several sections are placed with the base of " No. 1 

 Cretaceous " upon the same horizontal line. 



Throughout the series, what first strikes the observer is the marked 

 absence of limestones, and the great predominance of sandstones. 

 Limestones exist, it is true, and, considering their relative thinness, are 

 very persistent, forming good horizons to refer other* beds to, but they 

 are wholly subordinate to the arenaceous element. 



THE TRIAS. 



General characters. — Eesting directly upon the smoothed, though often 

 in-egular, surface of the Archaean rocks, and forming the lowest member 

 of the exposed stratified formations, is a series of sandstones which varies 

 in thickness from over 1,G00 or 2,000 feet to possibly as little as 400 feet, 



