170 
GEOLOGICAL REPORT-THIRTY-FIFTH PARALLEL. 
the ancient extinct volcano of Mount Taylor, that is seen some distance to the north. These 
streams of lava which spread over the bottom of the valleys are exactly similar to the streams 
from volcanoes in actual activity, and, like these, are destitute of vegetation, and give to the 
country, where they are found, an arid and desolate aspect, named by the Mexicans, very appro¬ 
priately, Mai Pais. 
Near the culminating point of the Sierra Madro the Trias is replaced by the carboniferous 
limestone ; then, for a distance of twelve miles, the rocks are eruptive granite, gneiss, and mica 
schist. Beyond, on the western declivity of the Sierra, comes the carboniferous again, the beds 
of the Trias, and finally the white and yellow sandstones of the Jurassic, with streams of vol¬ 
canic lava in the valleys. Inscription Bock, and the whole mesa that extends nearly to Zuni, 
are formed of the Jurassic rocks. A stream of lava spreads itself in the valley of the Ojo Pes- 
cado, and terminates three miles from the pueblo of Zuni. 
The valley of the pueblo and river of Zuni is of Triassic rocks, formed here, as in the prairie, 
of sandstone and red clay, with dolomite and gypsum. On the plateau which we cross from 
Zuni to the Colorado Chiquito, and from there till we arrive at a distince of five or six miles 
from the secondary Cones of the great volcano of the San Francisco mountains, we are con¬ 
stantly upon the Trias. These rocks are nearly horizontal upon the table-land, after having 
dipped to the east and west near the Sierra Madre, where the.y are very much upheaved. As 
we approach the Bio Colorado Chiquito, the strata incline to the north at a varying angle whose 
maximum is fifteen degrees; the heads of the strata looking towards the Sierra of Mogoyon, 
which is seen forty miles to the south. 
Shortly after quitting the Colorado Chiquito we found here with the last beds of the red clay 
of the Trias, and, in concordant stratification, a magnesian or dolomitic limestone, with very 
regular strata from half a foot to one foot in thickness. Several beds contain fossils badly pre¬ 
served ; among which I recognised, however, a Nautilus , a Pteroceras, and a Belemnites. This 
formation, which is placed between the Carboniferous and the Trias, corresponds, without 
doubt, to the magnesian limestone of England, and is a new member which I add to the series 
of secondary rocks in North America. 
This magnesian limestone has only four miles of extent in the place where we crossed it, and 
disappears beneath lava and volcanic ashes. I have observed it further to the west, and it 
appears also to occupy eastward one of the lesser chains of the Sierra de Mogoyon. 
From the Sierra of San Francisco to Cactus Pass the geology of the country we passed through 
is very complicated, on account of the immense extinct volcanoes, which have covered with their 
lavas and basaltic streams the sedimentary and granitic rocks that primitively formed this 
region. The study of this part of our route was rendered still more difficult by the snow-storms, 
that covered the ground with an immense white sheet during nearly the whole time of our ex¬ 
ploration. 
I will only say, in general, that there are four or five large extinct volcanoes over this space, 
the lagest being that of San Francisco, which is twelve thousand feet above the level of the sea. 
In places where the lava does not entirely cover the ground we find magnesian limestone, the 
sandstone of the coal-measures, and the carboniferous limestone—the last containing fossils 
in abundance, the principal ones being the Productus semireticulatus and Punctatus , and the Spi- 
rifer striatus. 
These stratified rocks are upheaved, and dip generally to the north-northeast, following several 
lines of dislocation which belong to the chain of mountains called Sierra de Mogoyon, or Sierra 
Blanca. In several places, and especially at Pueblo creek, beds of Old Bed sandstone are seen 
below the lower carboniferous, and in contact with the gneiss and granite, similar to the Old 
Bed of the Catskill mountains. 
This system of dislocation of the Sierra of Mogoyon, the direction of which is east east-south 
and west west-north, is anterior to the apparition of the Bocky mountains and the Sierra Madre, 
and I place it at the end of the Triassic period, and before the deposite of the Jurassic. 
