18 
GEOLOGY 
of blue clay at the base, then yellowish sandstone; and, finally, the summit is again a very com- 
pact, white siliceous limestone. 
In the whole, this formation of the Llano Estacado does not exceed four hundred feet in thickness. 
This formation is not limited to the Llano , but it forms the summits of all the plateaux that are 
for the valley of the Rio Grande del Norte to recruit our strength and forget the privations and hard- 
ships of the Prairies. 
Obliged to confine myself to a single excursion, I chose an isolated hill, on the left of our road, 
six miles from our camp N° 49, and which we called Pyramid Mount, (Latitude 35°, 01', 16"; longitude 
103°, 58'.), its shape being that of a quadrangular pyramid. The north side of Pyramid Mount is en- 
tirely precipitous, perpendicular as a wall, without any particle of vegetation and showing all the strata; 
it is impossible to desire a clearer or better geological section. The height of the bluff, where the beds 
outcrop is five hundred feet. I give below the section as 1 observed it in a short examination of only- 
four hours duration. 
Pvramid Mount. 
Feet. 
(i. White limestone . . 2 
F. Yellow limestone . . 50 
E. Blue clay 30 
D. White sandstone . . 25 
C. Yellow sandstone . 80 
B. White sandstone . . 8 
A. Grayish blue clay . 1 Keuper. 
a. Zone of the Gryphoea dilatata var. 
Tucumcarii and Ostrea Marshii iti di- 
vision E Blue clay, 
From the base nearly^ half way up, the first two hundred feet are composed of strata of Variegated 
Marls, red, green and white, having the same appearance as the upper part of the Keuper in the quar- 
ries of Boisset near Salins, France. A bed of grayish-blue clay (A.), one foot thick, forms the last strata 
of A'ew Red Sandstone, and is in contact with a white very fine-grained sandstone (B.), which is eight feet 
thick, and belongs to the Jurassic formation. Above there is an enormous mass , eighty feet in thickness, 
of very hard fine-grained sandstone (C.) of a light yellow color and cut by cleavage perfectly perpendi- 
cular like a wall. Beds of white sandstone (D.) are superposed ; they are very fine, soft and easily dis- 
integrated by the action of the atmosphere; at the foot of each bed little heaps of sand are seen which 
result from this decomposition; the thickness of the beds is twenty-five feet. Then comes clay (E.) of a 
slightly grayish-blue color and subschistose structure, thirty feet thick. 
In this blue day, six inches distant from the white sandstone, I found a zone {a.) of Gryphcea 
not more than three inches thick, but the specimens are so abundant that they arc in contact with 
each other. The Grypheea that I had collected at the foot of the bluff and on the ascent, though rolled 
and worn, had struck me as resembling in shape the Gryphaea dilatata of Oxford and of the Vaches Noires 
