30 GEOLOGICAL SURVEY OF THE TERPJTOKIES. 



geological structure, in luy report for 1869, l3ut calling attention to' the 

 more detailed work of Mr. Mar vine and Doctor Peale. I may say here 

 that soon after leaving the base of the mountains we have what are 

 usually called the plains, and the slope is generally to the eastward, becom- 

 ing less and less to the Mississippi Eiver. Ten miles from the foot-hills 

 the general elevation is from 5,000 to 6,000 feet above the sea ; the surface 

 is rolling, and in traveling from north to south we gradually rise to the 

 summit of one water-divide and descend into the valley of some stream 

 which flows from the mountains. These streams are usually, on an 

 average, about ten miles apart, and from one margin of descent to the 

 opposite side it is from five to eight miles. These valleys have been 

 worn out slowly by causes which will be noted in another portion of this 

 chapter. In a former part of this chapter I described in detail the series 

 of sedimentary beds from the granite nucleus of the mountains eastward 

 into the plains. From Cache a la Poudre far southward to the Arkansas 

 the plains are underlaid by some portion of the Lignitic group. The su- 

 perficial deposits are so extensive and the exposures of the underlying 

 beds so rare that it will always be difficult to work out the succession of 

 the beds in detail. It is most probable that Denver is underlaid by the 

 lower portions of the group, and that the divide far southward between 

 the waters of the Platte and Arkansas is composed of upper beds of this 

 group. The aggregate thickness of the entire group, as seen in Colo- 

 rado, cannot be less than from 3,000 to 5,000 feet. On a geological map 

 attached to my final report of Nebraska I represented the fresh- water 

 lake deposits as overlapping the Lignitic and extending far southward 

 toward the Arkansas. It is probable that some portion of that area is 

 occupied by the Lignitic group alone. That the Lake or White Eiver 

 group covered a larger area southward along the base of the mountains 

 is most probable, but that has been worn away so that the i^resent area 

 occupied by them would extend around to the southeast, much as shown 

 on the map. I am not positively certain that they extend south of the 

 line of the Kansas Pacific Eailroad. We know, however, that they cover a 

 large area south of the Union Pacific Eailroad. Iilast of Denver the Lig- 

 nitic beds must extend two hundred miles without interruption. It is 

 possible, also, that somewhere in this great area basins occupied by more 

 modern Tertiary deposits may yet be found. It is probable that the Car- 

 boniferous group does not occur continuously, as colored on the map, 

 from the line of the Union Pacific Eailroad to Colorado Springs. Near 

 the sources of Fountain Creek both the Carboniferous and Silurian 

 beds appear, and then southward, interrupted here and there, the former 

 group occurs along the flanks of the mountains. The more carefully- 

 prepared maps, which will be made hereafter by the survey, will correct 

 all local details, though in a general way this map was very correct. 



I have already alluded to the fact that the belt of uplifted sedimentary 

 ridges became narrower at the Big Boulder. Between the Big Boulder 

 and Clear Creek all the strata were lifted up at a high angle. Even 

 the Lignitic strata are close to the granites, and stand at high angles, 

 varying from 40° to 70°. South of Clear Creek the belt is not wide, but 

 it expands somewhat wider now and then, but south of Platte Caiion 

 hugs the mountains even more closely, the inner or lower strata lying 

 high on the sides of the foot-hills. Nowhere along this belt, from Big 

 Boulder to Plum Creek, do we see any traces of undoubted Carbonif- 

 erous beds. The lowest strata exposed next to the granites vary some- 

 W'hat in color and texture; the prevailing color is brick-red, or reddish 

 brown ; but in some places the lower beds are composed of a rather 

 coarse conglomerate cemented quite closely together. In the vicinity 



