G. K. Gilbert—The Colorado Plateau Province. 87 
Mountain building by eruption.—The studies, which the Pla- 
teaus afford in the phenomena of eruption, are scarcely less 
interesting and important than those of uplift and downthrow; 
ut they have received less attention up to the present time. 
It happens that a number of extinct eruptive mountains stand 
near the cafions of the Colorado River. The country about 
them has suffered and is suffering rapid denudation, and not 
only are their bases nearly free from detritus, but their flanks 
are so deeply scored, and their summits are so degraded, that 
their internal structure is exhibited. Those that are best 
known have been found to be composed chiefly of sedimentary 
strata, protected from denudation by the superior durability of 
the eruptive rocks with which they are associated. 
In the U’-in-ka-ret mountains Major Powell found a mass of 
undisturbed strata, which had been preserved from erosion by 
a mantle of lava, while the surrounding country was degra 
‘More than a thousand feet. The eruptions were extended 
through a long period of time, and the successive outflows 
mantled the flanks of the surviving strata almost as thor- 
oughly as they did the summit, so as to give the appearance, 
at first glance, of a range made up entirely of volcanic matter. 
In the Henry Mountains the strata are not undisturbed, but 
have been lifted into a number of bubble-shaped domes, one 
for each individual mountain of the group. Each dome has 
been fractured at top, and divided by fissures radiating from 
the center toward the sides, and all the fissures have been filled 
by molten rock. Moreover the strata have in many places 
cleaved apart, and lava sheets have been interleaved with them. 
ubtless the intrusion of these dikes and sheets was accom- 
panied by extrusion, but none of the extruded masses appear 
to have survived the subsequent erosion. The mountains as 
they stand are simply domes of curved strata, each traversed 
by a plexus of crystalline dikes. 
Similar in structure to the Henry Mountains are Navajo 
Mountain, Sierra la Sal, and Sierra Abajo. Mount San Fran- 
cisco, and perhaps Mount Taylor, are related to the Uinkarets. 
This enumeration includes but a small portion of the volcanic 
mountains of the district, and to the two types of structure 
Mentioned, several others might be added. But the mountains 
of the Uinkaret and Henry types are most favorably situated 
for study, and at the same time diverge most widely in charac- 
ter from those with which geologists are already familiar. 
Stratigraphy.—In the stratigraphy of the Plateaus attention 
has thus cae confined to caailde that are chiefly of 
local importance, but a thorough study of the phenomena 
Which are accessible can hardly fail to throw light on the 
