Vol. VIII] DUMBLE— GEOLOGY TAMPICO EMBAYMENT AREA 131 



"The Mendez marls consist of a very uniform deposit of 

 gray to blue shales and marls. In regions of steep folding 

 these often show bold jointing near the surface. There is 

 practically no change in their lithological character from top 



to bottom They average from 2,000 and 3,500 feet 



in thickness. A few irregular beds of sandy limestone are 

 reported in this formation, but they are not persistent." 



So far as can be judged from the reports now available, 

 south of these exposures between Micos and Valles, beds hav- 

 ing the characteristics of the San Juan formation have only 

 been observed as narrow detached bodies lying along the 

 border of the Sierra Madres. 



In a great many places through the region south of the 

 railroad blue shales are found underlying the yellow clays, 

 sands, and limestones of the Oligocene, and prior to the dis- 

 covery of Tertiary fossils in such a shale at Alazan, this en- 

 tire series of blue shales was supposed by us to be a continu- 

 ation of those of the valley to the north and to be of similar 

 age to the Papagallos. 



The only blue shales which were originally thought to be 

 later were found by Cummins in the region about Chiconte- 

 pec and while no fossils were found, on account of the litho- 

 logic similarity of the interbedded limestones and sandstones 

 to those on the Salinas River, these beds were tentatively re- 

 ferred to the Eocene. 



While there is a similarity of color existing between the 

 Alazan and Chicontepec beds on the one side and the Papagallos 

 on the other, they differ both in composition and in weath- 

 ering. 



The Papagallos is prevailingly clayey, weathering first into 

 slaty particles and finally to very black sticky soil, while the 

 others are usually more sandy, are frequently micaceous, and 

 often weather to grayish or yellow sandy soils or loams. The 

 prevailing dips of the Papagallos are northward and west- 

 ward and in places at rather steep angles, while the Tertiary 

 usually dips eastward at lower angles. 



South of Aquismon the scarp of Tamasopa limestone bends 

 sharply eastward nearly to the Tempoal River, a distance of 

 over 40 miles. It there bends southeastward again. The 



