THE STRUCTURAL CLASSIFICATION 589 



the dome, water on the outskirts and oil between, generally at the point 

 of greatest change in rate of dip. This relation appears true in all fields, 

 though in some saline dome fields the dip is so steep that water is pumped 

 in large quantities from the same well as the oil. Moreover, gas is not 

 such an important commercial factor in quaquaversal structure as in cer- 

 tain other types of oil fields. An evidence that the structure constitutes 

 the concentrating factor is brought from the Louisiana and Texas fields, 

 where hundreds of wells have been drilled away from the saline domes, 

 with a result that no oil was found. In this class of fields, as in the 

 monoclinal and anticlinal types, the oil seems to have been widely dis- 

 seminated in the porous strata and ultimately accumulated at favorable 

 points where the regularity of the dip is locally interrupted, or where 

 held in by water, gas, dikes, faults, or by pinching out of the porous 

 strata. 



CLASS V— CONTACT OF SEDIMENTARY AND IGNEOUS ROCKS 



Subclass V(a) — Contact of sedimentaries with volcanic plugs. — The 

 importance of this class is attested by the fact that the close association 

 of seepages with the volcanic plugs of Cerro de la Pez and Cerro de la 

 Dicha was the direct cause of the discovery of the Ebano field, the first in 

 Mexico. A large number of seepages also occur surrounding volcanic 

 plugs at Cerros Chapapote and Las Borrachas near Juan Felipe; Cerros 

 Palma Eeal and Cacalote near Potrero de Llano ; Cerro Pelon near Solis ; 

 Mata de Chapapote, at Caracol and Apachiltepec on Tlacolula, and many 

 other places. 



The principal function of igneous intrusion in the accumulation of 

 oil is believed by De Golyer*^^ to have been the formation of channels 

 through which the oil has been able to migrate into the overlying forma- 

 tions and even to reach the surface. A secondary and relatively unim- 

 portant function of intrusion is believed to have been the formation by 

 brecciation and metamorphism of reservoirs capable of containing oil. 



Subclass V(b) — Contact of sedimentaries ivith dikes. — Many of the 

 Mexican seepages occur along dikes of basaltic rock in the Tertiary sedi- 

 ments. The Tampalachi seepage near Panuco, Mexico, occurs on a con- 

 cealed dike. Other instances of seepages along dikes occur at Tamijuin, 

 Acala, Chapapote in the San Jose de las Rusias hacienda, and one mile 

 southeast of Cervantes. This last is illustrated in figure 14. 



Subclass V(c) — Contact of sedimentaries with intrusive beds or lacco- 

 liths. — Few cases are known with certainty where oil occurs below intru- 



«8 E. De Golyer : Econ. Geol., vol. 10, 1915, p, 661'. 



