no 
LACCOLITHIC STRUCTURES 
The arrangement of these mountains along a definite line is 
especially noteworthy not only because of its novelty among 
laccoliths but because of the fact that in the underlying ancient 
geologic structures of the region a reason therefore is found that 
probably affords a satisfactory explanation for all laccolithic 
structures., 
Since the first recognition of the laccolithic nature of Eos Cer- 
rillos hills and the Ortiz Mountains little seems to have been added 
to our knowledge of the area except through Prof. D. W. John^ 
son’s account.^ Messrs. Lindgren, Gratton and Gordon ^ who 
spent considerable time, in the district, appear to have entirely 
missed the essential structural characteristics. W. T. Eee,® 
although recognizing the location of the Tijeras fault at one point 
near the head of the Tijeras canyon, fails to draw its proper trend, 
or to comprehend its great extent, stratigraphic importance, and 
its curious tectonic significance. 
Linear Disposition of Laccolithic Sierras. All descriptions of 
laccolithic mountains make little or no attempt to connect the areal 
arrangement of the intrusions with lines of acquired geologic 
structures. In fact all accounts agree in assigning them a chance 
location. A novel feature of the New Mexican laccoliths is a 
recognizable regularity of their situation. The four groups there 
represented are arranged along a nearly straight line which runs 
northeast and southwest, ’or at an angle of about 45 degrees with 
the axes of both the southern Rockies and the nearest Basin 
ranges. 
This linear disposition of the New Mexican laccoliths is hardly 
fortuitous. An arrangement of this description calls for some 
kind of tectonic dependence. Such actually seems to be the case. 
After proof is forthcoming in one specific instance, the query 
arises whether it is not so in other instances and whether it is not 
really a relationship that is determinable in all laccolithic masses. 
There is now strong presumptive evidence that this is so. One 
main reason why the relationship was not earlier recognized is 
the fact that in accordance with a bias of the hypothetical demand 
it was the custom to draw the ground-plan of a laccolithic group 
on the basis of circles having as centers the highest mountain 
points. Were the Sierra del Oro laccoliths projected in this way 
1 Columbia School of Mines Quart., Vol. XXIV, p. 303, 1903. 
2 Prof. Pap. U. S. Geol. Surv., No. 68, p. 163, 1910. 
3 Bull. U. S. Geol. Surv., No. 471, p. 574, 1912. 
