98 GEOLOGY. 
VALLEY OF THE RIO GRANDE. 
The geological structure of the immediate borders of the Rio Grande presents the same ele- 
ments, with similar complications, as that of the region lying west of it. From the observa- 
tions made by our party, and by those who have crossed the continent south of our line, it 
appears that the same general description will also apply to a belt of country east of the Rio 
Grande, extending to a great distance north and south, and reaching as far eastward as the dis- 
turbing influence of the Rocky mountain ranges is felt. The generalities of the geology of all 
this great area may be given in few words, but the strata are so much disturbed by numerous 
and interrupted lines of upheaval that the work of investigating its local features would be 
one of infinite labor. Considerable time was spent by our party on the Rio Grande, at and 
above Albuquerque, and the geological observations made then include many local details, which 
it is scarcely necessary should be given, as they would swell the dimensions of this report 
without adding, in any material degree, to its value. 
This region has been so often visited and fully described that I shall confine myself to a brief 
synopsis of its geology, presented mainly for the purpose of comparing it with that of the 
country before passed over, and of noticing some errors which have been promulgated in refer- 
ence to it. 
The valley of the Rio Grande in the vicinity of Albuquerque is for the most part occupied by 
sandstones and shales of Cretaceous age, continuous with or resting upon those spread over so 
much of the surface in Western New Mexico. 
A little north of Albuquerque, on the west side of the river, a large area is occupied by a 
mesa, composed of Cretaceous rocks and the underlying red and white indurated marls, precisely 
like that northeast of Laguna. Like the mesas bordering the Rio San José, this is capped with 
a thin layer of basaltic trap, and it is quite probable both are but portions of a continuous 
plateau, wholly covered by a lava flood from Mount Taylor. 
Picturesque escarpments, formed by the cut edges of this mesa, border the valley on the west 
side near the pueblos of San Felipe and San Domingo ; thence northward, for many miles on the 
right bank, the same formation prevails, in some places the trap entirely concealing the sedi- 
mentary strata. 
On the east side, near Albuquerque, the valley is bounded by the Sandia mountain, of which 
full descriptions have been given in the report of Captain Whipple. The structure of this 
mountain is visible even from the river; the escarpment of Carboniferous limestone, that forms 
its summit and covers its eastern slope, being plainly distinguishable from the underlying 
anite. 
From Albuquerque to Algodones the surface is generally composed of alluvial soil or beds of 
drift from the Sandia mountains. The only rock seen in place is the red calcareous sandstone, 
or indurated marl, of the Upper Marl series. This forms low hills back of Manzana and 
Bernalillo. 
Between Algodones and Gualisteo creek is a table land principally formed of the white and red 
indurated marls. In one locality is an exposure of white concretionary tufaceous rock with 
bands of cream-colored nodular limestone; they are without fossils, as far as observed, and of un- 
certain date, but the lithologicaf characters lead me to suspect that they are of fresh water 
origin and Tertiary age. . 
: On Galisteo creek the strata are considerably disturbed, but the exposures are very full and 
interesting. 
_ The strata visible in the north bank of the stream where we crossed it are— 
1. Cretaceous sandstones, yellowish or greenish white, and shales with beds of lignite. 
oe 
= ‘Red and white marls, all somewhat indurated with silicified wood. 
8. Soft red sandstones and shales of the Salt group. 
