HAYDEN.] 



GEOLOGY COLORADO EANGE. 19 



number of ridges, wliicli are called iu the country " Hogbacks.'' Some- 

 times the inclination of these ridges from the granite nucleus outward 

 to the plains is not more than from 10° to 25^, gradually diminishing 

 until the Lignitic strata assume an entirely horizontal position, so far as 

 can be detected by the -eye. Again, the entire group of strata will be 

 crowded into a space of a mile or less, and stand at a nearly or quite a 

 ♦vertical position, but in suddenly passing from a very highly inclined 

 position to an apparently horizontal one on the plains, we can see that 

 however much the Metamorphic rocks which form the nucleus or body 

 of the great mountain-ranges may have been folded by the shrinking of 

 the crust, the sedimentary beds have been simply lifted up in a nearly 

 or quite vertical manner. I have often stated in previous reports the 

 belief, founded on most satisfactory evidence, that the sedimentary 

 strata formerly extended uninterruptedly across the area now occupied 

 by the Metamorphic mass of the mountain-ranges; that on the east and 

 west slopes can be found the broken portions inclining in opposite direc- 

 tions, but showing most clearly that the intervening portions had been 

 worn aw^ay in the process of upheaval. The sections across the range 

 will illustrate this statement most clearly. So far as can be seen at the 

 present time the process of upheaval has been very slow, long continued, 

 and uniform in its action. In many instances the sedimentary group 

 seems to have resisted the central force, and thus the strata were broken 

 off, and the edges turned up very abruptly over a very narrow belt or 

 zone, as at Golden City, and many other localities south of that point. 

 Again, the uplift seems to have influenced the strata for a long distance 

 from the Metamorphic nucleus into the plains, as at Cache a la Poudre, 

 ■where the inclination of the beds is nowhere very great, and gradually 

 diminishes eastward until they become quite horizontal ten to twenty 

 miles from the axis of power. In many instances the force from below 

 seems to have acted so nearly vertically that only the Lignitic and per- 

 haps a portion of the Cretaceous strata are exposed on the flanks of the 

 nucleus, and thus beds are broken off so abruptly that the detached por- 

 tions are thrown past a vertical and incline away from the mountain 

 mass; in other words, the gxeat Metamorphic mass that forms the nucleus 

 of a range has been pushed up so directly vertical that the operation 

 did not materially disturb the sedimentary group except immediately 

 around the flanks. This i)henomenon is not uncommon over the eastern 

 portion of the Eocky Mountain district. As the details of the geology 

 of the different districts are more fully worked out these points will 

 appear more clearly to the mind of the reader. It will be seen that the 

 geological structure of the Eocky Mountains is comparatively simple in 

 its general outlines, but that in its details it is remarkably complicated. 

 In the third annual report of the survey, season of 1869, I noted an 

 interesting feature in the structure of the mountain-ranges along the 

 eastern flanks, from Cheyenne to Santa F6. 1 directed attention to the 

 curious anticlinal ridges that seemed to extend down from the main 

 range and die out in the plains. Thus the great range or mountain 

 mass that fronts the plains from our north line to Mexico is made up of 

 a vast number of smaller ranges grouped together ; and while the great 

 mass, as shown on our maps, has an aggregate trend about northwest 

 and southeast, the front range from Laramie Peak to Santa Fe seems to 

 strike about north and south. If we examine the eastern flanks of the 

 range we shall find, from point to point, smaller ranges or spurs exteaid- 

 ing down from the main mass toward the plains, with a trend about 

 northwest and southeast, and soon dying out, leaving between the end 

 of the spur or minor range and the main mass a broad open valley, which 

 forms the sources of the more important streams. Most excellent illus- 



