18 Gilbert's Geology of the Henry Mountains. 
“Vermilion Cliff” and “Shinarump” groups, and has a thick- 
ness of 2,930 feet; and the Upper Carboniferous. These strata 
in the mountains are intersected by dikes of trachyte rising 
from a mass of trachyte below. The dip of the strata varies 
from zero at top and at base to various angles between, being 
even 80° in some of them just above the base; but the rocks 
beneath the plain around them are horizontal. 
e quaquaversal dip in the strata appears to indicate, as Mr. 
Gilbert states, that the dome-like elevations were produced 
through force acting directly beneath each; and, from the position 
of the trachyte, the natural inference is drawn that the force was 
connected with the eruption of this igneous rock. In the ideal 
sections given, one of which is here reproduced, a mass of 
trachyte is represented occu- 
ing an oven-shaped cavity, 
with the strata bulged upward 
above while horizontal below. 
-| The trachytic mass is called a 
laccolite, from o¢ cistern, 
in science of names of kinds of 
minerals and rocks, the mod- 
ified form of the term, Jaccolith (analogous to monolith) would 
be better, and is used below as essentially Mr. Gilbert’s.) 
The laceoliths are flat, or nearly so, below, as was found to 
be true at eleven localities, showing that it had taken the form 
of the surface on which it rested. The thickness or height is 
sometimes over 8,000 feet; and the breadth is, on an average, 
seven times the height, but in one case only three times. The 
trachyte dikes which rise from it are much more numerous 
than might be inferred from the ideal section ; and they often 
come up between the beds as well as intersect them. The sand- 
stone above and below the trachytic mass, or adjoining the 
dikes, is usually more or less altered by the heat for a thick- 
ness of a foot or more. The erosion which has reduced the 
original domes to deeply gorged mountain peaks and ridges 
has in some of them exposed part of the interior laccolith so as 
to show its original surface, while in one the whole stands bare, 
but much eroded through the action of waters. 
The chamber occupied by the laccolith was in all cases made 
along a shaly layer in the formation, where the cohesion was 
least. The trachyte is a compact porphyritic variety, wholly 
destitute of any trace of cellules. ‘here are faint indications 
of three or four successive beds in some of the masses 
In further explanation of these peculiar mountain structures, 
the following sentences and illustrations are cited from Mr. 
Gilbert’s Report. 
Ideal section of a Laccolith. 
