206 
LACCOLITHIC GENESIS 
made at a time when the theoretical necessity was yet unforseen. 
With the recognition of the genetic association of the two phe¬ 
nomena a host of other inexplicable facts is rendered readily 
understandable. 
It may be that in crossing the old fault-line an orographic 
flexure creates conditions which enable the fractured supra-lacco- 
lithic strata to float upon the viscous lava, thus apparently con¬ 
firming in a way Gilbert’s contention of the relative densities 
as a controlling cause of laccolithic phenomena. 
The remarkably feature of the the faulting it that the dis¬ 
placement may be downward beneath the level of the laccolithic 
mass and upward above the lava zone. This phase is particularly 
well displayed in the San Ysidro and the Tuertos laccoliths. In 
other instances the displacive movement may be cumulative as 
shown by the Ortiz and Los Cerrillos masses. These apparently 
diametrically opposite results along the same crustal rupture is 
probably to be ascribed to the fact that the fault-plane is a warped 
surface and the stress is thus torsional. The torsional character 
of neighboring faults that are parallel to the potential orographic 
axis is best indicated by the courses of great dikes which traverse 
the country from the laccolithic centers. Some of these dikes 
have a linear extent of a score of miles. One of them, near the 
O’Mera coal mine, seven miles south of Ortiz Station, is repre¬ 
sented in the annexed diagram (figure 13), and plainly exhibits 
marked lateral movement as well as vertical displacement along 
its sides. 
Localization of Orographic Potentialities, Beyond the topo¬ 
graphic boundaries of a folded mountain range the flexing of 
strata in diminishing amplitude extends far on either side. Soon 
after passing the southern state-line of Colorado and in the lati¬ 
tude of the Sierra del Oro, the Rocky Cordillera abruptly ends. 
As a conspicuous anticlinorium it pitches beneath the general 
plains-surface of the Mexican Tableland, never again to reappear 
either as a relief form or as a tectonic entity. Two great parallel 
folds mark this terminus of the Rockies. They are locally desig¬ 
nated as the Las Vegas and Santa Fe ranges. Minor parallel 
flexures are present to the eastward and to the westward. The 
western corrugations extend a distance of more than 30 miles 
from the Santa Fe axis. These minor flexures cross the great 
