34 
LACCOLITHIC MOUNTAINS 
dependence of this class of mountains upon prior geologic struc¬ 
ture. The shape of laccolithic masses is found to be cuneiform 
instead of lenticular; and thus at once does away with the blis¬ 
ter idea. Quite essential appears to be the presence of crustal 
lines of weakness. The magmatic swelling or localization of 
laccoliths is discovered to be a direct function of orographic po¬ 
tentialities. 
« 
In seeking an immediate cause for this laccolithic intrusion 
Gilbert did not lose sight of certain mechanical shortcomings of 
his explanation. These he sought to overcome by appealing to 
certain associated factors, which, however, later. Whitman Cross 
showed to be both unnecessary and not demonstrative as such. 
Dana got over the difficulties by brushing aside all considerations 
except simple hydrostatic pressure and with this feature alone 
regarded the Gilbertian hypothesis complete. This is doubtless 
one of the main reasons why from a mechanical angle leading 
European Geologists have so persistently challenged the American 
view of laccolithic intrusion. At the same time Old World writ¬ 
ers on the theme oflfer no alternative theory to take the place of 
the one which they seek to discredit. Through the results of close 
inspection of certain laccoliths of northern New Mexico the chief 
objections which were raised against the Gilbert view seem to be 
fully met. Controlling tectonic factors which all describers of 
laccoliths have missed thus appear to supply the long sought 
desiderata. 
As a primary consideration in order that a laccolith be pro¬ 
duced rather than any other form of volcanic manifestation it 
appears that the instrusive mass shall have a particular tectonic 
setting. Profound''faulting is one of these prime factors. An¬ 
other is orographic flexing by which the rigidity of certain arching 
strata largely maintains the load of superincumbent materials. 
Probably the high viscosity of acidic magmas has an important 
but as yet uncalculated influence on events. The remarkable 
infrabasal structure which the New Mexico laccoliths reveal 
carries the inquiry a step more remote and explains the deep- 
seated cause of the major faulting, whereby an orographic prism 
is sustained by a sharp Pre-Cambrian arch, the rigidity of which 
is not even yet lost although the adjoining blocks on either side 
are allowed to slide down, as it were, the steep sides of the old 
flexure. 
