78 
GEOLOGY 
Blanca, which extends between the Sierra Madre, the Rio Colorado Chiquito, the Bill William Fork, the 
Great Rio Colorado of California, and the Rio Gila; or in other words, between 35° and 33° of latitude, 
and 108° and 114° of longitude, west of Greenwich. This system is composed of a great number of 
chains and parallel ridges, of which the general direction is N. 60° W. to S. 60° E. The highest points 
of these mountains are near the sources of the Rios Gila and Prieto, where they appear to bo 9,000 to 
10,000 feet above the sea-level. 
From the small number of observations I have been able to make in a rapid geological reconnais- 
sance of that region, I think these chains of mountains belong to a system of dislocations, which has 
ruptured and elevated the strata of the Upper New Red Sanstone, and ended the Triassic period of North 
America. 
The rocks which the Sierra de Mogoyon has brought to the surface are as follows : — a highly 
amphibolic granite forming the centre, then motamorphic quartzose rocks covered by strata of the Old 
Red Sandstone or Devonian; these strata are formed ol beds of very hard red sandstone, resembling 
the sandstone of Catskill in New-York State. Above we hnd the Mountain Limestone or Low'er Carboni- 
ferous highly developed, and containing numerous characteristic fossils of that series, then sandstone 
of the Coal-Measures or Upper Cai’boniferous, Magnesian Limestone or Permian, and, finally, numerous strata 
of Trias. The Jurassic series is deposited horizontally upon slightly elevated strata of Trias and in a discordant 
stratification, such as I have observed at Fort Defiance and the Chevelon Fork of the Colorado Chiquito. 
I think the dislocations which have affected the Upper Trias , containing the Coal seams near Rich- 
mond, Chesterfield County, in Virginia, belong to this system, as also the mountain chains extending 
between the Great Salt Lake and the Serpent river or Lewis’ Fork of the Columbia. 
The Shasta Range, forming the boundary between California and Oregon, and occupying all the 
country comprised between the Sacramento and Willammette rivers, and Capes Mendocino and Umpqua, 
has a direction coinciding precisely with the System of the Sierra de Mogoyon. 
10. Rocky Mountains and Sierra Madre system. — The Rocky Mountains and the Sierra Madre 
form, in the centre of the American continent a mass of rounded elevations following parallel 
lines, in some degree symmetrical. These bombements are often broken by long narrow lines, and then 
the eruptive rocks have forced themselves a passage through, elevating and dislocating the sedimentary 
rocks belonging to the Carboniferous, Triassic and Jurassic series. 
The general direction of these chains is on an average N. 15° W. and S. 15° E., and the time of 
upheaval is at the end of the Jurassic period and before the deposit of the Neocomian scries of America. 
The Carboniferous strata, and especially the Mountain Limestone are broken and upheaved, for they 
are found in contact with the eruptive and rnetamorphic rocks ; and I have seen the Mountain Limestone 
with its most characteristic fossils, even at an elevation of 12,000 feet above the sea-level. 
Having ascended one of the highest peaks of the Rocky Mountains in the vicinity of Santa Fe, in 
New' Mexico, the elevation of w'hich is 13,000 feet, — from that height, favored by the pure and light 
atmosphere peculiar to that region, I discovered a horizon having a radius of 150 miles, embracing 
many chains of the Rocky Mountains; such as the sierras de Manzana, de Sandia, deJemez, de Taos, 
do San Juan ; a part of the Sierra Madre ; groups of old and extinct volcanoes , and two sierras which 
run in the direction of the Rio Pecos. I have never seen, even from the summits of the Alps , the lines 
of dislocations so well determined , and their parallelism so visible, with the most neatly marked outlines. 
The ridges (dcaillements) and convexities forming this system of the two groups of the Rocky Moun- 
tains and of the Sierra Madre, occupy a breadth varying from 120 to 200 miles; the eruptive rocks do 
not appear at all the ruptures, and do not exceed a breadth of 12 or 15 miles, sometimes only ap- 
pearing for a space of two miles. 
In the Rocky Mountains, as in all great mountain chains, there are linos and accidents of dislo- 
cation anterior and posterior to the principal upheaval. Thus, the Placeres mountains south of Santa 
Fe, and the mountains east of San Pedro, have a direction and accidents of stratification, indicating a 
date anterior to the appearance of the neighbouring Sierra de Sandia. 
