Vol. VIII] DUMBLE— GEOLOGY TAMPICO EMBAYMENT AREA 115 



of the vast quantity of petroleum that has been developed in it 

 during recent years. It is geologically important not only on 

 account of the oil, but also because it furnishes the key to cer- 

 tain heretofore unsolved problems regarding the relationship 

 of adjacent land areas to continental growth. 



This area, some 300 miles in length, will not average 50 

 miles in width. Its greatest breadth, which is less than 100 

 miles, is found along the course of the Panuco River and its 

 tributaries, whence it narrows both to the north and to the 

 south. 



Physiography 



Topographically, the area as a whole is a plain sloping gently 

 gulfward. Along its western border are low ranges and ridges, 

 rarely exceeding 300 meters in height, caused by the strong 

 folding and faulting of the Cretaceous rocks together with 

 some of those of earlier Tertiary age which form its basement. 

 To the east of these its undulating surface is broken by hills 

 of erosion and by peaks of intrusive basaltic rocks. North of 

 the Panuco River these interruptions are less numerous than 

 they are to the southward. The most prominent remnantal 

 elevations are found in a series of peaks, mesas and ranges be- 

 ginning at Chicontepec and stretching northeastward to the 

 Otontopec range which ends near Tantima. This forms the 

 divide between the drainage of the Panuco and that of the 

 Tuxpam River. 



Between these two rivers are two intermediate coastal basins 

 which have been carved out and are drained by the Cucharas 

 and the Tancochin and a like service is performed by the 

 Cazones and Tecolutla for the area between Tuxpam and 

 Nautla. 



The principal drainage system north of the Panuco is the 

 Soto la Marina and between it and the Conchos River, along 

 which we found exposures of typical Gulf Coast Tertiaries, lie 

 the mountain masses of the Sierra de San Carlos and the 

 Sierra Cruillas of the Tamaulipas range. These mountains 

 extend westward to within 15 miles of the railroad south of 

 Linares, greatly narrowing the valley at that point. 



Much of the surface is covered by the dense vegetable 

 growth of the semi-tropics and for the most part the so-called 



