22 Gilbert's Geology of the Henry Mountains. 
ig of the ee is 1,200 feet, and its diameters are 6,000 
and 4,000 fee 
In the fc laccolith “the trachyte has a depth of only one 
thousand feet, but it lies so high with reference to the general 
degradation that it is a conspicuous feature of the topography. 
e edges of the laccolite are all eaten away, and only the cen- 
tral portion survives. All of the faces are precipitous. The 
cover of shale or sandstone has completely disappeared, and the 
aoe nts : 
cue ee eat 
aE we ei SOSA 
Jukes Butte, of the Ellen Cluster, as seen from the northwest. 
upper surface seems uneven and worn; but a distant view 
(figure 6) shows that its wasting has not rogressed so far as 
to destroy all trace of an original even surface. The eminences 
of the present surface combine to give to the eye which is 
aligned with their plane the impression of a straight line. The 
hill is loftier than the laccolite, for under the one thousand feet 
of trachyte are five hundred feet of softer rock which constitute 
its pedestal, and aon their vege undermine the laccolite and 
perpetuate its cli 
The laccoliths me described as occurring at different levels, 
between beds of the Carboniferous, Jura-Trias, and Cretaceous 
probably covered originally by at least 7,000 feet of strata 
exclusive of the Tertiary. But since the Tertiary, according 
to Mr. Gilbert, probably spread over the region, and owes its 
bsence only to subsequent denudation, the total thickness, 
since that of the ela a is 3,500 to 7, 000 feet, may have been 
much over 10,000 fee 
The facts brought out appear to sustain Mr. Gilbert’s conclu- 
sion that the mountains were made such, out of horizontal 
strata, by the ascending trachyte, which “insinuated itself 
between the strata, and —— for itself a chamber by lifting 
all the superior beds.” 
The steps in the early when of the history are given as follows 
on page 95 of the Report. 
