LACCOLITHIC GENESIS 
211 
magmas has an important but yet uncalculated influence on events. 
The horst structure of the Sierra del Oro carries the matter 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 yet lost, while 
the adjoining blocks on either side are allowed to slide down, as 
it were, the steep sides of the old flexure. 
In the course of the uncovering of laccolithic masses through 
erosion different groups display different stages of advancement. 
The Ortiz Mountains represent a state of depletion in which 
the covering is completely removed and the unearthed body is so 
profoundly dissected that the mass is almost nearly cut through 
to its foundations. San Ysidro still retains part of its cover. 
The laccolithic masses of Los Cerrillos, in New Mexico, and 
Mount Marceline, in Colorado, are even less exposed. Henry 
Mountains seem to be only in the initial stages of revelation. 
Some of the outlying mounds of the last mentioned group have 
merely hypothetical existence. Considering this stage alone it is 
small wonder that Gilbert hit upon the simplest explanation, and 
could not at the time at which he made his observations fancy 
any structural dependence of the all but hidden masses. His 
blister explanation was as simple and as natural as could be, and 
when he came to make the model of the Henry Group and a res¬ 
toration of its features the quaquaversal mounds were a necessary 
consequence of his hypothesis. 
Later realization of such possible complications of the problem 
as the Sierra del Oro structures present perhaps accounts for 
Gilbert’s strong desire, just before his demise, to review, the 
structures which he had made so famous, before passing final 
judgement on the theme of his early efforts. 
It may be noted in this connection that close inspection of the 
original ground-plan of the Henry Mountains, in the light of 
the Sierra del Oro structures, discloses a possible line of dislo¬ 
cation, like in the case of the New Mexico example, parallel to 
the axis of the great Water Pocket flexure, a short distance to 
the west of the Henry Group. Probably the Henry Mountains 
should be revisited, and their ground-plan redrawn along tectonic 
lines, if possible, for surely these mountains do not have mere 
fortuitous setting. Perusal of the published description of Henry 
