LACCOLITHIC GENESIS 
203 
TECTONIC SETTING OF LACCOLITHIC GENESIS 
By Charles Keyes 
Incompetency of Unaided Hydrostatic Pressure. In striving 
adequately to account for the cause of the laccolithic phenomena 
presented by the Henry Mountains Gilbert ^ makes last recourse 
to the type of rock, the relative densities of the intruded mass 
and the invaded strata, and the general law of hydrostatics. Com¬ 
menting upon this effort Dana ^ is satisfied that Gilbert’s expla¬ 
nation is complete without reference to the differences in rock 
density, hence inferring that simple hydrostatic conditions are 
all-sufficient. Cross also, in the course of his discussion on the 
cause of laccoliths, is inclined towards a similar opinion, and cites 
in evidence the occurrence on the brink of the Grand Canyon 
of normal volcanoes from which lavas fell in cascades 2000 feet 
into the gorge. “These fissures were evidently not made by a 
force like that of the laccolithic eruption to the north.” Although 
it may be conceded that with so powerful a forced movement in 
in the lavas no other cause than that of simple and unaided hydro¬ 
static pressurei is needed for flow to any level of the invaded 
strata at which a fissure might terminate the fact remains that the 
laccolithic form of intrusion is far too infrequent in occurrence 
to have this agency regarded as the controlling, or sole, factor. 
On the other hand are the opinions of Suess,^ Daly,* Jagger,® 
and others, that the magma during intrusion is essentially passive. 
The actual association of laccolithic bodies with definite crust¬ 
al rupture and with the severe localizing of some masses in the 
paths of marked orographic flexing, as amply shown in the 
Sierra del Oro, seems clearly to demonstrate beyond all peradven- 
iTieology of Henry Mountains, p. 75, 1877. 
2 Am. Jour. Sci., (3), Vol. XIX, p. 24, 1880.' 
3 Sitzungberichteder Wiener Acad., Bd. CIV, p. 52, 1895. 
4 Journal of Geology, Vol. XIII, p. 503, 1905. 
5 Twenty-first Ann. Kept. U. S. Geol. Surv., Pt. iii, p. 172, 1901. 
