672 V. R. GARFIAS 
the change in the nature of the beds. The oil could then have 
migrated along fissures toward the upper beds, aided by the action 
of water, and collected in the upper Cretaceous limestones and 
shales, to be concentrated in the favorable zones created by the 
igneous intrusions. Such migration had necessarily to take place 
after the deposition of the Cretaceous-Eocene cap-rock. 
The accompanying figures summarize graphically the writer’s 
conception of the effects of the igneous intrusions on the accumu- 
lation of oil in northeastern Mexico. Fig. 1 shows an intrusion 
which reaches the surface. In this case some oil may percolate 
through the shales, forming seepages near the basalt outcrop, and 
wells of varying productiveness may be located at comparatively 
short distances from the outcrop. If the intrusions penetrated 
the Cretaceous-Eocene shales to a point below the surface (Fig. 2), 
these would be arched above the basalt, thus affording a dome- 
shaped reservoir in the shales, which might prove commercially 
productive for a comparatively short time. If, however, the intru- 
sions perforated only a portion of the series of limestones and 
shales at the top of the Cretaceous limestones (Fig. 3), the re- 
sultant dome of porous and fractured material, capped by a thick 
cover of impervious shales, would form an ideal reservoir, which, 
if tapped, would readily account for the tremendous gushers char- 
acteristic of this region. Any and all of these conditions may 
exist, and it is probable that, with certain modifications, all do 
occur at one point or another in the pou of the Mexican 
coastal plain herein mentioned. 
August 5, 1912 
