SURVEY OF COLORADO AND NEW MEXICO. 25 



2. The red bed passes up into a massive reddisli-gray ratlier fine sand- 

 stoDe— 20 feet. 



3. Then comes a thin layer of fine bluisli-brown sandstone — 2 feet; 

 then the bluish limestone — 4 feet. 



4. Then about twenty-five feet of ashen clay, with six to ten feet of 

 blue cherty limestone, with some partings of clay. 



5. About two hundred feet of variegated clays. 



6. A bed of quite pure limestone, blue, semi-crystalline — four to 

 eight feet. The grass prevents definite measurements, and all the beds 

 vary in thickness in different places, as well as in dip, Avhich is from 60° 

 to 80°. 



7. This intermediate space is covered over with a loose drab yellow 

 sand, doubtless derived from the erosion of the edges of the beds beneath, 

 which are supposed to be Jurassic. There is one bed of limestone about 

 two feet thick, similar to that before described. All these limestones 

 appear to contain obscure fragments of organic remains. 



8. A nearly vertical wall of sandstone; dip 60° to 65°. This bed is 

 formed of massive layers, in all, one hundred and fifty feet thick or 

 more, and is composed largely of an aggregate of small water-worn 

 pebbles of all kinds. Most of the pebbles are of metamorphic origin, but 

 some of them appear to have been derived from unchanged rocks. 

 There are also layers of fine-grained sandstone. The prevailing color is 

 a rusty yellow and light gray. Most of the sandstones in this country 

 are of a rusty yellow color ; No. 1, cretaceous. 



9. A broad space, three hundred to four hundred feet, grassed over. 

 The slope is complete, but it is uudoubtedly made up of the sands and 

 sandstones at the base of the cretaceous group. 



10. A fine sandstone passing up into a close compact flinty rock. This 

 is a low ridge, appearing only now and then above the grassy surface. 

 The slope then continues down to the stream which flows through the 

 synclinal valley about a mile wide, and then we come to the grassy slope 

 on the mountain side inclining east again. A little below this point the 

 creek cuts through the sandstone and black clays of ^o. 2, conforming 

 perfectly to the wall of sandstone No. 1. 



It is now well known that the great Eocky Mountain system is not 

 composed of a single range, but a vast series of ranges, covering a width 

 of six hundred to one thousand miles. There are also two kinds of 

 ranges, one with a granitoid nucleus, with long lines of fracture, and in 

 the aggregate possessing a specific trend ; the other has a basaltic nucleus, 

 and is composed of a series of volcanic cones or outbursts of igneous 

 rocks, in many cases forming those saw-like ridges or sierras, as the Sierra 

 Nevada, Sierra Madre, &c. Along the eastern portion of the Eocky 

 Mouutains, from the north line to New Mexico, the ranges with a gran- 

 itoid nucleus prevail. Each one of the main ranges is sometimes split 

 up into a number of fragments, which locally may vary somewhat from 

 a definite direction, but the aggregate trend will be about northwest 

 and southeast. 



As I have before stated, each one of the main ranges seems to me to 

 form a gigantic anticlinal with a principal axis of elevation, and the lower 

 parallel ranges descending like steps to the plains, or to the synclinal 

 valley. If, for example, we Avere to study carefully one of the minor 

 mountain ranges, as the Black Hills 'of Dakota, or the Laramie range, 

 where the system is very complete and regular, we should find a central 

 granitic axis, and on each side a series of granitic ridges parallel with it, 

 and in the aggregate trending nearly north and south. And ou the east- 

 ern portion of the anticlinar, the east side of the minor ridges slopes 



