GEOLOGY. 
99 
appear strange that the shale of this "butte could lie in the same horizontal plane with the 
limestone of the hills, and that it should have resisted devastation so long. 
An analysis of atmospheric air, which I made in the neighborhood of Fort Atkinson, may 
also properly find a place here. The mean of three analyses executed with all care after the 
method of Prof. v. Liebig—absorption of the oxygen hy a solution of pyrogallic acid in caustic 
potash—gave: 
Nitrogen... 79.09 
Oxygen. 20.91 
100 
This shows, as was to he expected, that the composition of the air on the high prairie does 
not differ from the well known invariable composition of atmospheric air elsewhere. 
On entering the mountains we find a white, fine-grained, very hard sandstone, torn, frac¬ 
tured, and upheaved to nearly a vertical position by plutonic rock. The steep slopes of the 
mountains are covered with fragments of a white silicious rock, which in some places are of con¬ 
siderable size, forming blocks of twenty and more feet in diameter. The hills on the foot of 
the higher mountains are covered with drift rocks of all kind—quartz, porphyry, sandstone, 
&c.; even hypersthene rock is occasionally met with. About fifteen miles west of the canon 
through which we entered the mountains, sandstone, and a red shale lying under it, are nearly 
vertically uplifted by a trachytic porphyry, which seems to have given to those mountains 
their peculiar shape and elevation. This porphyry consists of a brownish gray, rough base, in 
which a great number of crystals of feldspar and crystals of black mica are imbedded, forming 
a rock of great beauty. The crystals of feldspar and mica are mostly perfect, and of the size 
of one to two tenths of an inch; the latter are hexaedrix prisms, with basal cleavage. This 
rock, which forms the ridge from the trail in Sangre de Cristo to the route laid down by Cap¬ 
tain G-unnison through that pass, seems to be of a very changeable character. East of the 
ridge the base of it is of a purer gray, the number of feldspathic crystals is less, and the 
numerous mica crystals are smaller, sometimes almost of microscopic size; but the basal 
cleavage is still easily detected. West of the ridge, the mica seems to have passed into tour¬ 
malin, the crystals of feldspar being scarce. The rock contains sometimes tolerably large 
crystallizations of a zeolitic substance, which for its physical properties and crystal form, as 
well as behavior before the blow-pipe, must be considered as stilbite. The predominating rock 
in the Sangre de Cristo valley is a feldspathic granite, passing gradually into gneiss on the 
right bank of the creek, the gneiss supporting a hard shale, sandstone, and a bluish brittle 
limestone. The latter belong perhaps to that class of non-fossiliferous transition rocks lying 
under the silurian system,* and the existence of which on this continent has been recognised by 
several distinguished geologists. 
In the neighborhood of Fort Massachusetts there are indications of iron ore, and even faint 
indications of cobalt ore, but I had neither the time nor the means to follow them up. 
In the sandy, and for the most part sterile, San Luis valley, I looked in vain for some 
section or outcropping of the rock lying under the soil. During many days’ travelling, this 
valley presents nothing but sand; and it was not even possible to ascertain the character of 
the under-soil. A striking and very curious illustration of the abundance of sand in this valley 
is found in an isolated row of sand-hills opposite Roubideau’s Pass. They are from 500 to 600 
feet high, running for several miles in a direction from northwest to southeast. It is very 
probable thqt some solid rock is buried under this sand, having been the cause of its accumu¬ 
lation. Along the foot of the White mountains (Sierra Blanca) we found numerous boulders 
of igneous rocks which do not seem to correspond with the rocks of the neighboring mountains. 
* The cambrian system, as distinguished from the silurian system by its age and organic remains, is not recognised any longer 
by geologists. Comp. Murchison, in Quarterly Journal Geology, soc. VIII, 1852. Murchison’s Siluria, 1654. 
