LACCOLITHIC GENESIS 
207 
Sierra del Oro horst at an angle of about 45 degrees. It is at 
the intersection of the two that the laccoliths are situated. Four 
in succession are exposed and possibly others remain yet undis¬ 
covered. 
The location of these laccoliths with reference to well defined 
lines of geologic structure is hardly fortuitous. In itself the 
four-time repetition of identical circumstances argues strongly 
for direct genetic association of the several phenomena. In the 
theoretical consideration of the peculiarities of the New Mexico 
exemplifications, in the mathematical analysis of the tectonic prob¬ 
lems presented, and in the satisfaction of the most urgent tectonic 
demands an adequate raison d'etre for laccolithic genesis seems 
to have been reached. 
Stratigraphic Horizon of Laccolithic Intrusion. The Sierra 
del Oro is particularly instructive at this time because of the 
fact that by it are supplied some very definite and pertinent data 
bearing directly upon the actual depths at which typical laccolithic 
intrusions take place. Two wholly distinct and unrelated lines 
of testimony are presented. In character one trend of evidence 
is strictly stratigraphic. The other has to do with the physical 
conditions attending the genesis of normal contact 'deposits of 
the metallic ores. 
From casual consideration of the great vertical section of geo¬ 
logic formations of the region, representing a thickness of up¬ 
wards 10,000 feet above the base of the Mid Carbonic limestones, 
the chief horizon of intrusion, it might be expected that this figure 
stands for the actual volume of overburden strata. There are 
several qualifying features. At least two great erosion intervals 
intervene. One of these appears to have been at the beginning 
of Cretacic time, and the other at the beginning of Tertic time. 
Along the belt of the Sierra del Oro horst the normal Carbonic 
column 3000 feet thick is reduced on account of Cretacic erosion 
to 300 feet. Early Tertic erosion cuts the Cretacic section of 
3500 feet to one-half. 
Concerning the physical conditions attending the formation 
of contact-ore deposits the Ortiz and Tuertos Mountains are 
especially referred to.'^ In the production of certain types of 
metallic, ores distance form the surface of the ground is fre- 
7 Economic Geology, Vol. VI, p. 365, 1909. 
