TO JUNCTION OF GRAND AND GREEN RIVERS. 21 



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

 valleys of the draining .streams. Below them are exposed strata containing Orlliix'nta 

 Hiitlirtirtiltiui J'roi/tirfii:-; C(i/l/<ntiii<ni,t<.<i ; spines of a species of Archceocidaris, regarded 

 by Professor Swallow as identical with A. V(.'rncit(/i(ii/(i, King; a small Athi/ris, and a 

 lili/inchiiiii'lla ; 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 fossil iferous; containing great numbers of Myalina perattenuata, Monotis (Pseu- 

 (loinoiKiti.s), I/dint/, Hnh'ri'lliK /><irr<t, and many other species described by Meek and 

 Hayden, who collected largely at this locality. 



COTTONWOOD CREEK TO WALNUT CHEKK. 



THE GYPSUM FORMATION. 



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

 surface, and are not distinctly recognizable at any western' point niton the Saute F(5 

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

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

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

 fills 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 geologist and 

 almost every trayeler 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 lithological char- 

 acters from base to summit, and in different localities, it has 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 of 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 Gypsum 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 counec- 



