Vol. VIII] DUMBLE— GEOLOGY TAMPICO EMBAYMENT AREA 135 



three feet thick. Some of the sandy beds have a strong petro- 

 leum odor while tarry black residues are frequent along frac- 

 tures and fault planes. This residuum is of a brittle texture and 

 disintegrates on burning. In other places it will take the 

 slickenside markings of the surrounding walls, thus assuming 

 an extreme similarity to lignite. 



Overlying this there is a long stretch of coarse limestones 

 about 450 feet thick. This limestone is brown in color, is fos- 

 siliferous in places, showing Niiniinulites, sp. Tiirritella, sp. 

 and Cardium, sp. It also contains some sandy beds and carries 

 small pebbles of rounded black chert and sandstone. 



Overlying this we have another series of alternating cal- 

 careous sandstones and shales which carries some conglomer- 

 ates locally. The harder beds in this series seem to have a 

 predominance of ripple marks. 



Jeffreys states that the Tanlajas formation follows south- 

 ward from this point, along the front of the Tamasopa lime- 

 stone outcrop, through the State of San Luis Potosi into Vera- 

 cruz and Hidalgo. He says nothing whatever of its relation 

 to the San Juan (San Felipe) or Papagallos (Mendez) and, as 

 he was fully familiar with those formations a few miles to the 

 north, it can be taken for granted that he considered this en- 

 tirely different and later. While Jeffreys refers this to the 

 Oligocene, it is probably the northward extension of the Eocene 

 beds existing in like relation to the Tamasopa farther south- 

 ward. Cummins considers it of similar age to the Chicontepec 

 beds west of El Xuchil. 



Apparently, these beds become more arenaceous as we go 

 south from Tanlajas and San Pedro, and the limestones dis- 

 appear. The most of the beds reported are marls overlain by 

 flaggy sandstones and bluish shales with few fossils except 

 nummulites. 



Sixty-five miles southeast of Aquismon, and some fifty miles 

 west of Tuxpam, Cummins and Sands found a series of beds, 

 the lowest members of which were seen at the crest of an anti- 

 clinal ridge on Chicontepec Mountain, a mile and a half east of 

 the town of Chicontepec, at an elevation of about 3,200 feet. 

 The beds are composed of yellowish brown sandstones, some 

 being two feet in thickness and containing weather-worn boul- 

 ders, inclusions, or segregations of a very hard steel gray sand- 

 stone. The boulders seemed to carry some carbonaceous mat- 



