EFFECT OF IGNEOUS INTRUSIONS ON OIL 667 
phenomena occur throughout the area examined. Among the 
localities where oil seepages are abundant might be mentioned 
La Pez, La Dicha, Chijol, and Santa Margarita, near the Eban 
field; San Geronimo, La Merced, Rancho Abajo, Monte Alto, 
and Los Higueros, between the famous Dos Bocas and Casiano; 
Casiano, Cervantes, Tres Hermanos, Tinaja, Ojo de Brea, Chapo- 
potillo, Monte Grande, Moralillo, Cerro Azul, Juan Felipe, Las 
Borrachas, Piedra Labrada, and Cerro Viejo, between the producing 
fields of Casiano and Potrero del Llano. 
The intimate association of the volcanic intrusions and some of 
the producing fields is so apparent as to be unquestionable, and the 
problems in this connection relate chiefly to the effect of the basalt 
on the structure and texture of the intruded beds which resulted 
in their becoming capable of storing large amounts of oil under 
tremendous gas pressure. 
The general conception of the form of these intrusions is that 
of a more or less irregular cone, in a normal position, the vertex 
of which may or may not reach the surface. Were these con- 
ditions fulfilled in the Mexican coastal plain, a well drilled near: 
an outcrop of basalt would eventually strike the side of the cone 
and thus preclude further progress. This assumption is certainly 
not corroborated by actual experience in the fields, as many of 
the best producers are located in very close proximity to intrusions, 
and in some cases the basalt has been penetrated for several feet 
and the well continued to the oil reservoirs below. 
In order to account for these discrepancies the writer performed 
a rather crude experiment, as a result of which were obtained some 
interesting data which seem to throw considerable light on the 
behavior of the intrusions and their effect on the intruded beds. 
Assuming the thickness of the uniform Cretaceous-Eocene shales 
as 3,000 feet, and that of the average intrusion as 1,000 feet, it was 
thought that the effect of the Jatter as it penetrated through 
the uniform series of shales would in some ways be parallel to 
that of a nail driven through an unbound book of a thickness 
three times as great as the diameter of the nail. The experiment 
was tried while the book was resting, first on a board, then on a 
folded blanket, and again on the open end of an inch pipe, and the 
