TO JUNCTION OF GRAND AND GREEN RIVERS. 21 



Permian magnesian limestones occupy the general surface, but arc cut through bythe 

 valleys oi the draining streams. Below them are exposed strata containing Orthisina 

 urribraculum Productus Calhounianics ; spines of a species of Archaocidaris, regarded 

 by Professor Swallow as identical with A. VerneuiHana i King; a small Athyris^ and a 

 UhynchoneUa ; all of which belong rather to the Carboniferous than to the Permian 

 fauna Near Cottonwood Creek the Upper Magnesian limestone, or true Permian, is 

 highly lossilit'ei'ous: containing great numbers of Myalina perattenuata, Monotis (Pseur 

 domonotis)) Haumi, Bakevellia i>u>T(t, and many other species described by Meek and 

 Ilavden, who collected largely at this locality. 



COTTONWOOD CKKEK TO WALNUT OREEK. 



THE GYPSUM FORMATION. 



On the west side of Cottonwood Creek, the Permian limestones pass beneath the 

 Surface, and an; not distinctly recognizable at any western point upon the Saute Fe 

 road. They are succeeded by a series of reddish-yellow and white indurated marls, 

 forming a part of the greal " Gypsum formation," which is so conspicuous a feature; in 

 the geology of the Indian Territory, New Mexico, and Western Texas. This group 

 tills the interval between the Permian strata, which I have described, and the base of 

 the Cretaceous system: including representatives of perhaps portions of the Permian, 

 the Triassic, and -Jurassic formations of the Old World. The magnificent exposures of 

 this series which abound in New Mexico, have been noticed by every • ffeolOffist and 



almost every traveler who has entered that country. It will be seen by reference to 



the reports upon the geology of the Southwest made to the (Government bythe writer 



or others, that this formation is everywhere characterized by great poverty of fos- 

 sils, and for this reason, as well as from the general similarity of its lithologieal char- 

 acters from base to summit, and in different localities, it lias been hitherto impossible to 

 separate it by satisfactory dividing-lines, or to determine with accuracy the equivalence 

 of any of its parts with the different formations which it may be supposed to repre- 

 sent The study devoted to this group of strata by the writer, while connected with 

 the party under the command of Lieutenant Ives, was not wholly fruitless, but it must 

 be confessed that, as far as regards the determination of the parallelism oi' its sub- 

 divisions with the strata to which they have been referred, it enables him rather to say 

 what they are not, than what they are. The observations made upon this formation in 



our recent explorations of the country bordering the San Juan and Upper Colorado 

 Rivers — where it is very largely developed — will be detailed in the subsequent chap- 

 ters of this report, and it is hoped that they will serve to throw some additional light 

 on this difficult and perplexing subject. 



The materials composing the Ovpsiun formation are usually SO soft that in a, 



country well supplied with rain and covered with vegetation they present few satisfac- 

 tory exposures, and are even usually wholly concealed from view. This is the char- 

 acter of the district now under consideration, and it is only here and there that the 



traveler can obtain even a glimpse of its geological substructure. It is evident, how- 

 ever, from the limited space occupied by the outcrop of this group, taken in connec- 



