20 Gilbert's Geology of the Henry Mountains. 
greater depth of strata. From the base of the arch there have 
been worn 3,500 feet of Cretaceous, and from 500 to 1,500 feet 
of the Jura-Trias series, which is here about 3,000 feet thick. 
From the summit of the arch more than 2,500 feet of the Jura- 
Trias have been removed. 
“The strata exposed high up on the mountain being older 
than those at the base, and the dip being everywhere directed 
away from the center, it is evident that the mountain is sur- 
rounded by concentric outcrops of beds which lift their escarp- 
ments toward it.” 
“The laccolite of Mount Ellsworth is not exposed to view, 
but I am nevertheless confident of its existence—that the visi- 
ble arching strata envelop it, that the visible forest of dikes 
and dike and sheet with laccolites, in other mountains of the 
same group.” 
The Hillers laccolith ‘is the largest in the Henry Mountains. 
Its depth is about 7,000 feet, and its diameters are four miles 
and three and three-quarter miles. Its volume is about ten 
cubic miles. The upper half constitutes the mountain, the 
lower balf the mountain’s deep-laid foundation. Of the portion 
which is above ground, so to speak, and exposed to atmospheric 
degradation, less than one half has been stripped of its cover of 
arching strata. The remainder is still mantled and shielded by 
sedimentary beds and by many interleaved sheets of trachyte.” 
‘* All about the eroded (south) face of the mountain the base is 
revetted by walls of Vermilion and Gray Cliff sandstone, 
strengthened by trachyte sheets. At the extreme south, these 
stand nearly vertical (80°), and their inclination diminishes 
gradually in each direction, until at the east and west bases of 
the mountain it is not more than 60°.” “The same beds which 
form the revet-crags on the southern base constitute also some 
of the highest peaks. Since these rest directly upon the lacco- 
lite, it is assumed that the next lower beds of the stratigraphic 
series form its floor.” ‘It is noteworthy that wherever the 
sedimentaries appear upon the mountain top they are highly 
metamorphic, But in the revet-crags [upturned Jura-T'rias 
sandstone about the south side] there is very little alteration.” 
_ The Mount Hikn Cluster (map, p. 17), having a diameter 
from north to south of more than ten miles, contains, if rightly 
understood, no less than sixteen laccoliths “in the spurs and 
foot slopes and marginal buttes” about the central crest. A 
view of the western flank of the mountain is shown in figure 
5. In this view, there are recognized, in front of the highest 
