122 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Se«. 



generally used, is restricted to the beds of the portion of the 

 section which are characterized by the presence of rudistes. 

 These are typically seen in Micos canyon and at the Choy 

 grotto which also illustrates the cavernous condition so preva- 

 lent in this limestone. 



The Tamasopa limestone is rather fine-grained, compact, 

 creamy to gray in color, and most usually massive. It is often 

 crystalline in structure and in places it is dolomitic. 



Between the Tamesi and the Tuxpam rivers the Tamasopa 

 limestone appears to be the principal oil producing formation, 

 while south of the Tuxpam valley it has not been found in any 

 of the producing wells drilled up to this time. 



Villarello. describing the beds of the Meso-Cretaceous lying 

 north of the railroad line between Tampico and San Luis 

 Potosi, says : 



"The Meso-Cretaceous is made up of limestones of a gray- 

 ish color in heavy beds with a strike about 18 deg. northeast 

 and dip of 31 deg. to the northwest. These limestones are 

 strongly folded and faulted and constitute a great portion of 

 the Sierra Madre Oriental which extends from the Tula dis- 

 trict passing through the western portion of the southern and 

 central districts of the State of Tamaulipas and afterwards 

 enters the State of Neuvo Leon. 



"The Tanchipa range rises to the west of Ebano and 



is made up of limestones and shales of Meso-Cretaceous and 

 Neo-Cretaceous age. These beds extend toward the south 

 and are exposed in nearly the whole of the petroliferous region 

 of Aquismon." 



Of the continuation of these deposits south of the railroad 

 he says : 



"The older sedimentary rocks (of the Aquismon region) 

 are heavy beds of a grayish colored limestone, fossiliferous in 

 some portions, especially in the neighborhood of Choy grotto 



"These limestones constitute the Meso-Cretaceous of the 

 region, and only the limestones in the vicinity of Xilitla prob- 

 ably belong to the Eo-Cretaceous. 



"The Meso-Cretaceous outcrops at the following places, 

 from the northwest of Xilitla through Tampachal and Pubuche 

 in the Temapache Mountains, to the west of Tocomon, Aquis- 



