SURVEY OF COLORADO AND NEW MEXICO. 23 



feature of this sandstone. It forms a portion of the £]jronp which I have 

 called trausitio!), or IsTo. 1. They are certainly beds of passage from 

 well-marked cretaceous to the Jurassic, and the lower portion being- 

 almost invariably a pudding-stone, they may well mark the boundary 

 between the two great periods. In many places along our route this 

 group forms lofty perpendicular escarpments, varying from thirty to sixty 

 feet in height, indicating a considerable thickness of the massive sand- 

 stone. For fifteen miles w^e can pass along behind this hog-back ridge par- 

 allel with the mountains, through a most beautiful valley with fine grass, 

 and over an excellent natural road. On our left are the upturned 

 edges of a ridge capped with ISTo. 1, i)assing down into the limestone and 

 ashen marly clays of the Jurassic, with a few feet of the red sandstone 

 at the base, while the valley, which may be three hundred to five hun- 

 dred yards wide, is composed of the worn edges of the loose red beds of 

 the triassic, and on our right are the variegated sands and sandstones of 

 the formation. 



South of Cache a la Poudre there seem to be but two principal ridges 

 between the transition group No. 1 and the metamorphic rocks, 

 although at times each one of these ridges will split up into a number of 

 subordinate ridges which soon merge into the main ridge again. In 

 most cases the inner ridge includes all the red beds proper, and there is 

 a well-defined valley between it aud the metamorpliic rocks, but some- 

 times the sedimentary beds flank the immediate sides of the metamor- 

 phic ridge. Through these ridges are openings made by the little streams 

 which issue from the mountain's side. Sometimes these openings are cut 

 deep through to the water level, and at other times for only a few feet froiu 

 the summit. Sometimes there is a stream of water ilowing through 

 them, but most of them are dry during the summer. These notches in 

 the ridges occur every few hundred yards all along the foot of the 

 mountains. , 



The cretaceous and tertiary beds generally form several low ridges 

 which are not conspicuous. The principal ridge outside, next to the 

 plains, is composed of the limestones of No. 3, which is smoothly 

 rounded and covered with fragments or chips of limestone. Betw^een 

 this and the next ridge west, there is a beautiful concave valley about 

 one-fourth of a mile wide. The line between the upper part and the foot 

 of the ridge proper is most perfectly marked out by the grass. The east 

 slope of this ridge is like the roof of a house, so steep that but little soil 

 can attach to it, and in consequence of this it can sustain only thin grass 

 and stinted shrubs. These ridges are sharp or rounded, depending upon 

 the character of the rocks of which they are composed. Cretaceous 

 formation, No. 3, yields so readily to atmospheric agencies, that the ridges 

 composed of it are usually low and rounded, and paved with chipped frag- 

 ments of the shell limestone. The harder sandstones give a sharpness of 

 outline to the ridges which has earned for them the appellation of "hog- 

 backs," by the inhabitants of the country. In No. 3 I found Os-frea con- 

 gesta very abundant, and a species of Inoeeramus identical with the one 

 occurring in the linjestone at South Boulder, and the same as the one 

 figured by Hall in Fremont's Report, Plate IV, Fig. 2, and compared 

 with Inoeeramus involittus, (Sowerby,) page 310. The lower part of No. 



3, containing the Inoeeramus, is a gray marly limestone, which passes up 

 into a yellow chalky shale, which "weathers into a rusty yellow marl that 

 gives wonderful fertility to the soil, while the dark shales of Nos. 2 and 



4, as well as the rusty arenaceous clays of No. 5, are distinctly revealed 

 at dif[^"ereut localities. The light-colored chalky limestones of No. 3 are 



