204 
LACCOLITHIC GENESIS 
ture that conditions other than simple hydrostatic pressure alone 
are often, if not always, involved. 
In the original consideration of the Henry Mountains it is 
manifest that essential features and factors are entirely overlooked 
and that in the light of later observations these laccolithic masses 
need further critical examination. 
Inadequacy of Relative Rock Densities. In the description of 
the Henry Mountains Gilbert lays particular stress upon the 
relative densities of the intruded lavas and the invaded strata. 
“The coincidence of the laccolithic structure with certain types 
of igneous rock is so persistent that we cannot doubt that the rock 
contained in itself a condition which determined its behavior.” 
That this circumstance alone is not sufficient to satisfy the equa¬ 
tions presented, is indicated by the fact that rock-type is a direct 
results of conditions of consolidation which take place after 
intrusion. As Cross ® so astutely points out, in both chemical 
and mineralogical composition the laccolithic rocks are identical 
with certain surface andesites. This point might be further em¬ 
phasized by noting that the identity extends not only to the lacco¬ 
lithic mass and surface flows but to the associate dykes, sheets 
and other appanages. 
In order that a laccolith may form it is evident that specific 
gravity of the magmatic mass is not competent to float the rock- 
prism above but that it must have had material assistance of some 
kind or other to produce the dome. This aid is believed to be 
found partly in rupture of the invaded strata thereby setting 
free one limb of a potential arch, and partly in local orogenic 
stress which initiates the flexing, the bow being maintained by 
competent strata in the rock-section. 
Essential Presence of Crustal Lines of IVeakness. The char¬ 
acteristic wedge-shaped outlines of many and possibly all lacco¬ 
liths and the close association of these intrusive bodies with ex¬ 
tensive fault-lines, as in the instance of the Sierra del Oro, is 
in itself strong presumptive evidence of a necessary genetic rela¬ 
tionship between this class of eruptive masses and planes of 
profound displacement. The attendant conditions are not unlike 
those which obtain when one moves over thin ice, the fluid be¬ 
neath not flowing out over the surface until some large crack 
6 Fourteenth Ann. Rept. U. S. Geol. Surv., Pt. ii, p. 239, 1894. 
