Gilbert's Geology of the Henry Mountains. 23 
“When lavas, forced upward from lower-lying reservoirs, reach 
the zone in which there is the least hydrostatic resistance to their 
accumulation, they cease to rise. If this zone is at the top of the 
earth’s crust "they build volcanoes; if it is beneath, ney build 
laccolites. Light lavas are more apt to produce voleanoe ; heavy, 
laccolites. The porphyritic trachytes of the Plateau pt oe 
produced laccolites 
he station of the laccolite being decided, the first step in its 
formation is the intrusion along a parting of strata, of a thin sheet 
of lava, which spreads until it has an area adequate, on the prin- 
ciple of the hydrostatic ston: to the deformation of the covering 
strata. The spreading sheet always extends itself in the direction 
of least resistance, and if ihe resistances are equal on-all sides, 
takes a circular form. So soon as the lava can uparch the strata 
it does so, and the sheet inlet a laccolite. With the continued 
addition of lava the laccolite grows in height and width, until 
nally the supply of material or the propelling force so far dimin- 
ishes that the lava clogs by congelation in its conduit and the in- 
flow stops.” “A second pai may take place either before or 
after the first j is solidified. It may intrude above or it may intrude 
beneath it; and oheetyatn0 has not yet distinguished the one 
from the other. In any case it carries dbs the deformation of 
The lifting force was thus due to the forced upward-flow of 
the lava; and it became able to overcome the resistance from 
the weight and cohesion of the rocks above by spreading into 
an opening beenieie the horizontal strata, and widening the 
area of press 
The force eae to the lavas at their source get 
was hence sufficient, it would appear, to throw a stream, in 
of friction along the passage and the density of the scaneeias | (at 
t 2°33 in fusion), for an unknown number of miles, up to 
the laccolith level; and sufficient at this point, further, to lift, 
in the case of the lowest of the laccoliths, a superincumbent mass 
of beds 10,000 feet thick (supposing the Tertiary at top 3,000 
feet of it) and 2°25 in average specific gravity (equivalent in 
pressure to 675 atmospheres) as a apmere of 5,000 feet. 
The Report offers no views the origin ‘of the propelling 
force. Whatever the source, it is possible that some accession 
of energy may have come from vapors derived by the ascend- 
ing lavas from subterranean moisture or waters encountered on 
their way up, though slight compared with the vast amount 
received from action below. 
