66 GEOLOGY. 
thickness of at least 5,000 feet, a vast quantity of lava has been poured, covering with a flood 
of melted matter the country for many miles around, and forming one principal cone with a 
thousand inferior ones; of which the more conspicuous have received distinct names. 
Little disruption of the stratified rocks attended this grand exhibition of volcanic force; and 
the formation of the mountain mass seems to have been effected entirely by the ejection of 
matter in a state of complete fusion, through narrow orifices of unfathomable depth. 
Comparatively few mountains have been wholly formed in this manner; probably none but 
those having the same isolated character with that under consideration. All the mountain 
chains which have come under my observation have been composed, in a great measure, of 
upheaved strata of a decided sedimentary character. Some of them more or less metamor- 
phosed. 
Lines of upheaval seem to mark linear fractures in the earth’s crust, and bear evidence of 
the action of lateral pressure as well as of elevatory force. In solitary mountains, on the con- 
trary, I have observed a marked absence of disrupted and metamorphosed sedimentary rocks, 
and the prevalence of masses of a purely plutonic character. The conspicuous summits which 
so generally mark the prominent angles in important mountain chains should apparently be 
included in the same category with isolated cones, as they also, as far as my observation has 
extended, are principally composed of ejected materials. 
The modern date of the later eruptions of the San Francisco mountain is attested by the 
remarkable freshness of some of the volcanic products which cover its slopes and base. Many 
of the secondary cones are distinctly crateriform, are composed of black or blood-red scoria, 
and are entirely destitute of vegetation; showing by all their surroundings that they have 
been in action, as it were but yesterday, and might be again to-morrow. 
Some of the currents of lava which have flowed down the sides of the San Francisco moun- 
tain belong so entirely to the present epoch that they have dispossessed still running streams 
from their beds, and now occupy their places in a congealed flood which seems but just arrested 
in its flow; as black and ragged, and as little affected by the action of the elements as slag 
‘fresh drawn from a furnace. 
As will be seen when we come to speak of it, the description given of the San Francisco 
mountain, its isolated volcanic cone, its recent lava streams, &c., will apply with almost equal 
force to Mount Taylor; and yet, beyond this similarity, there is perhaps nothing to connect them. 
The volcanic phenomena of modern date, exhibited on so grand a scale in the Cascade moun- 
tains, and so fully described by the writer, (Pacific Railroad Report, vol. VI,) together with 
those of the San Francisco mountain, Mount Taylor, the Raton, and a multitude of similar 
peaks scattered throughout the country embraced in the ranges of the Sierra Nevada and 
Rocky mountain: prove that the forces which have elevated these great systems have, at 
various points, c ntinued to act even up to the present time. The earthquakes that still pre- 
vail upon the Pacific coast, and the recent oscillations of level of which it bears record, are 
phenomena of similar import. With those before cited, they lead us to infer that the origin 
of the labyrinth of mountain chains, covering such an immense area in the far west, is not to 
be referred to one or two remote paroxysms, but to the operation of an incessant and long con- 
tinued action, commencing anterior to the palaeozoic period, and continuing to the present 
epoch; and that even now this shaken and everchanging region has not reached a period of 
repose. . 
LOCAL GEOLOGY. 
, Compe led 2 fare back at the caiion of Cascade river and pass around south of the San 
ae capes in order to cross the Little Colorado, we also retraced our geological steps 
ee ay ed to the strata exposed in the western edge of the high mesa, some fifty miles 
a0 uth 1 of the point where we first struck it. In following southward along the western border 
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