A SHORT NOTE ON THE OIL FIELDS OF MÉXICO 123 



show signs of great moveraents. The shale formation, as 

 seen in cuts made by rivers or in places where the gravel 

 and stuary material of an eroded peniplain which mantled 

 it is washed away, the shale, I says, shows only slight 

 gradient ondulations, monoclinal low ridges and ampie 

 vaults, never big or repeated foldings, neither great dis- 

 placements by faults. The latter as far as our general ob- 

 servation, are generally of very small importance. 



Dykes are practically unknown in the central oil lands 

 between the Tamesí and Tuxpan Rivers and besides the 

 necks and small lava streams there are some rare hills of 

 plutonic rocks like diorites and nepheline syenites. Further 

 north in the State of Tamaulipas and south to the middle 

 part of the State Veracruz, some sierras breaking up the 

 coastal plains are more complex geological structures. 



Local interior áreas very ilisturbed, certainly exist, but 

 have not been proved productive as yet. Highly distnrbed 

 shales are found also as a belt in the western margin of 

 the formation against the underlying limestone of which the 

 foothills and the eastern Sierra Madre mountains are 

 built up. 



The volcanic necks are seen scattered in the coastal plain 

 commonly as isolated cones or as small groups of hills; 

 the first type is wonderfully represented in the section be- 

 tween Chicontepec and Tuxpan, of the State of Vera- 

 cruz (1). 



We should not make the volcanic action responsible for 

 all the ondulating and gently bossy structure of the large 

 shale oil bearing formation, for everything tends to prove 

 that the main tectonic,'weak as it is, is prior to the volcanic 

 activity in the coast. The volcanic plug or pipe of ashy ancl 

 compact basaltic lava, of which the neck is the upper end, 

 has not produced any considerable disturbance of the shale 



(1 ) «Oil in the State of Veracruz. » Min. and Se. Press. Aug. 24th. 1907. 



