LACCOLITHIC MOUNTAINS 
29 
than a figment of the imagination. From what is lately gleaned 
from a consideration of the Henry Mountains region there ap¬ 
pears to be without doubt certain larger tectonic associations 
with the smaller laccolithic structures of which no mention is made. 
Although the ideal cross-section of the Henry Mountains may 
prevail in one direction in all but exceptional instances it seems 
never to occur in all directions in the same laccolith. In all like¬ 
lihood it is the stiffness of a diagram of the ideal conception that 
has done more than anything else to delay the ready and universal 
acceptance of the laccolith as a specialized expression of magmatic 
intrusion. Along with the discernment of well defined tectonic 
affinities of laccolithic structure the type or ideal form assumes 
outlines wholly different from that originally suggested. With 
the new theoretical conception the observed facts seem strictly to 
agree. 
Normal Asymmetry of Laccolithic Masses, In his graphic ac¬ 
count of that lacolithic group in western Colorado, known as 
the West Elk Mountains, Dr. Whitman Cross incidentally shows 
in a diagram that one of the principal elevations. Mount Marcel- 
lina, is, in cross-section, essentially a thick wedge-shaped mass 
instead of a symmetrically developed lenticular body. It does not 
seem to have dawned upon this writer that this might possibly be 
the normal shape of laccolithic bodies rather than the exceptional 
form. The same is true of the neighboring Anthracite Mountain. 
In the course of his theoretical discussion of the forms of lacco¬ 
liths Cross indicates that in the case of the irregular masses 
faulting of the strata once underlying might take place, and if 
^the rock column were very thick the break might pass upward 
into a monoclinal fold. 
In New Mexico all members of the Sierra del Oro group of 
laccoliths appear to be notably asymmetrical in shape. Careful 
inspection of other laccolithic mountains clearly discloses the 
fact that they too are not regularly lenticular in form as once 
supposed, but are really irregular or rather cuneiform in trans¬ 
verse outlines. The Tuertos Mountains of New Mexico for ex¬ 
ample, present the following structural aspects as represented in 
cross-section (figure 2). This diagram may be with full assur¬ 
ance taken as the ideal or type form of all laccolithic intrusions. 
15 Fourteenth Ann. Kept. U. S. Geol. Surv., pt. ii, p. 184, 1894. 
