24 Gilbert's Geology of the Henry Mountains. 
The idea in the commencement of the above citation—that 
hydrostatic pressure determined the level of the laccolith among 
the strata—is dwelt upon at length in the preceding pages of 
the Report. The author argues that the relation as to density 
between the liquid trachyte and the several associated stratified 
rocks, and between the latter among themselves (the several 
densities of which he gives), is the chief cause determining the 
level among the strata of the laccoliths; saying that the lava, 
ree to move upward or laterally, will intrude itself among the 
strata at a point ‘so placed that every combination of superior 
beds, which includes the lowest, shall have a less average den- 
sity, and every combination of inferior strata, which includes 
the highest, shall have a greater density, than that of the lava.” 
“If the fluid rock is less dense than the solid, it will pass 
through it to the surface, and build a subaerial mountain,” or 
“volcano ;” if more dense than the upper portion of the solid 
rock, the fluid will not rise to the surface, but will pass between 
the heavy and light solids, and lift or float the latter.” Cohe- 
sion in the rocks modifies the result; but, he says, neverthe- 
less, after discussing this point, “we are Jed to conclude that 
the conditions which determined the results of igneous activity 
were the relative densities of the intruding lavas and of the 
invaded strata; and that the fulfillment of the general law of 
hydrostatics was not materially modified by the rigidity and 
cohesion of the strata.” 
We refer to the report for a full explanation of this part of 
Mr. Gilbert's theory. To the writer, his explanation appears 
to be complete, without reference to this difference of density. 
With so powerful a forced movement in the lavas as the facts, 
if they are rightly interpreted, show to have existed, no other 
cause could be needed for a flow to the surface in case of an 
open channel, or for a flow to any level in the strata at which 
a fissure might terminate; and this is true, whether the lava be 
light or heavy. In fact heavy lavas, having a specific gravity 
of 2°85 to 3:1, make the larger part of modern volcanic cones, 
as well as of non-volecanic igneous outflows, and one of the 
lighter lavas—trachyte, of the specific gravity 2°61, by Mr. 
Gilbert's determination—made the laccoliths. 
These lighter lavas are adapted to the purpose because they 
are the least fusible of ordinary igneous a ts and they owe 
their difficult fusibility to the orthoclase feldspar which is the 
chief constituent, whose fusibility on Von Kobell’s scale is 
marked 5, while that of albite is marked 4, and of labradorite 
and volcanic augite, 8. Such lavas are hence easily chill 
and thicken greatly in the upper part of narrow fissures or of 
volcanic conduits, and it is for this reason that they have often 
made steep-sided domes over subaerial vents. 
