CHAPTER III • 



Eesume of the Geology along the Eastern Base of the Front or Colorado 

 Kange: Silurian, Carboniferous, Triassic, Jurassic, and Cretaceous Groups. 



The various groups of sedimentary rocks that occur along the eastern 

 slope of the Front Eange of Colorado from Canon City to Cheyenne 

 have been so often described in previous reports that it seems hardly 

 possible to add anything further of importance. We will be able, there- 

 fore, to do little more in this connection than to describe, with some care, 

 the maps, sections, and other illustrations which we have prepared for 

 this report. 



The '"Preliminary map of the eastern base of the Eocky Mountains" 

 will be found to explain itself to a great extent. The topography is 

 given with much detail and in a picturesque form, so that the relations 

 of the sedimentary to the granitic rocks are admirably presented. The 

 characteristic forms of the u Hog Back" ridges which have been so 

 often noticed, is clearly shown, as well as the en echelon features of the 

 minor mountain-ranges as they run out into the plains. 



The pictorial sections which accompany the map will serve to show 

 more clearly than we have hitherto done what we have denominated 

 the plain and mountain districts, as well as the abrupt transition from 

 one to the other. The plain country extends uninterruptedly from the 

 Missouri Eiver to the base of the mountains. The elevation above 

 sea-level at Kansas City is 764 feet ; at Denver, by way of the railroad, 

 639 miles to the westward, the elevation is 5,197 feet; showiug an aver- 

 age ascent of about seven (7) feet per mile over an apparently level, 

 treeless plain. Over this broad space the strata are very nearly or 

 quite horizontal in position, until within a few miles of the mountains, 

 where they are lifted up at various angles and the mountain-ranges 

 seem to rise abruptly out of the plains. The topography as well as the 

 geology of the plain country is remarkably simple, and it is only in a 

 narrow belt along the immediate foot of the mountains that it becomes 

 more varied and complex. The elevation of the great Front or Colorado 

 range carried up the sedimentary formations which originally rested on 

 its sides or summit, and the upjift seems to have been very nearly or 

 quite vertical. Whether these formations originally extended uninter- 

 ruptedly across the area now occupied by the mountain-ranges, is a 

 question which will be more fully discussed at some future period. That 

 this was the case in part, I am very confideut, but there are facts that 

 appear to disprove this statement in some instances. It seems proba- 

 ble that a portion of the Eocky Mountain range was outlined at an 

 early period; that it has grown, as it were, through successive ages up 

 to the present time. A careful examination of the map and the picto- 

 rial sections will enable the reader to understand more clearly the 

 remarkable belt of uplifted sedimentary beds along the immediate base 

 of the mountains. Although they seem to be more interesting and 

 picturesque in Colorado, yet these ridges occur to a greater or less 

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