78 GEOLOGY. 
All the strata of the groups I have described are conformable among themselves as well as 
to those of the Carboniferous rocks below and the Cretaceous above. 
Mr. Marcou fills the interval between the red sandstones of the Little Colorado and the 
Cretaceous strata of the Rio Grande with a series of rocks of much greater thickness, and pre- 
senting diagnostic characters quite different from such as came under our observation. As will 
be seen hereafter, there is reason to believe that the ‘‘ Jurassic rocks’’ of his classification are 
Cretaceous, and in reference to his subdivisions of the variegated marls I can only say that we 
failed to recognize them on our route. They may be plainly discernible in the region traversed 
by him east of the Rio Grande, but certainly are not so in Western New Mexico. His second 
Triassic group, of which the place is immediately above the red sandstones of the Little 
Colorado, is, according to his description, (Geology of North America, p. 11,) composed of beds of 
white and red clay, containing immense masses of gypsum, and traversed by veins of selenite, with 
numerous interposed beds of magnesian limestone, and frequent deposits of rock salt. To this 
division he assigns a thickness of fifteen hundred feet. In the various exposures of the strata 
overlying the red sandstones, which we examined in the country between the Little Colorado 
and Rio Grande, we found them to consist of highly colored marls, containing bands of magnesian 
limestone, most numerous near the base, but running through nearly the entire series. No beds 
of rock salt were observed, and the gypsum is most abundant in the upper part of the group, 
while the silicified wood is confined to the lower half—facts quite at variance with those cited 
by him as the grounds of his classification. This division Mr. Marcou considers, with perhaps 
as much reason as the first above the Carboniferous limestone, as the equivalent of a member 
of the European Trias; in this case of the Muschelkalk. 
The third division which Mr. Marcou has established he denominates the Variegated mar’s, 
and regards it as the equivalent of the Marnes Irisées of France, and of the Keuper of Germany. 
In this division he locates the petrified forest of Lithodendron creek, and considers it character- 
istic of the formation, whereas the marls containing the silicified wood of Lithodendron creek, 
and of the north side of the valley of the Little Colorado generally, rest directly upon the red 
saliferous sandstones, the greater number of fossil trees which they contain occurring within 
two or three hundred feet of the surface of that group. 
The intimate structure of the mesa bordering the Little Colorado on the northeast will be 
seen from the following sections. 
The bluff bounding the valley north of Camp 85, crossed when entering the Painted Desert, 
is composed of the strata enumerated below. 
Variegated Marls. 
Feet. 
1. Ochery sand, very soft; forming summit ....-..--- eeeese cece ce eeee teen tees eet 2 
Tei White Mark «oe Pees et oS Se cee ee coc ck co Piast reese 20 
a eerie White Ineonesinn UleNNe «662 ok pew oc oc pees Sa See wes sage pee seke os 11 
MM RUE WRAL so ok eins hn es Sst cae ew eek mses ees ute set 21 
5. Brey, ME OUIOT LiMCCL ONE <6 ay os ee bho sow ob soe cnc ces Ce owe ey a sneee tees 4 
ee ee Vee ne oer eee Vee anes were eee 40 
S. Petr Gime ee I es re ck ks ew bok 400 
A few miles further east, at the mouth of Cottonwood Fork, a continuation of the same mesa 
afforded me a somewhat more complete section, reaching higher into the series of variegated 
seri. It consists of the following elements: 
