Vol. V] DUMBLE— TERTIARY DEPOSITS IN NORTHEASTERN MEXICO JQl 



only ones so far found in beds we have recognized as Oligo- 

 cene. The following is a section of the deposits near Salitre: 



Feet. 



Coarse gray sandstone .- 20 



Yellow clay, fossiliferous ....100 



Hard yellow clayey limestone, fossiliferous 4 



Yellow sandy clay 10 



Yellow sandstone, fossiliferous 8 



Yellow clays and sands 10 



A range of hills known as the Martines which are similar 

 to those seen east of San Jose de las Rusias and of about 

 equal height is found here extending from Salitre southward 

 nearly to the Soto la Marina River. Along the Conchos 

 River the exposures of the Oligocene are of beds higher in 

 the series than the bulk of those of San Jose de las Rusias, 

 being represented in that region by the Pecten beds which 

 lie along its extreme eastern border. In the valley of the 

 Conchos the greenish clays and soft sands with their beds of 

 gj^psum, which are part of the Frio, are found as far east 

 as Tepetate and forming the body of the hills lying directly 

 north. Beds of the Oligocene are found not only overlying 

 these beds at this point, but stretching several miles westward, 

 showing a clear overlap to lower beds of the Eocene section. 

 What seem to be the lowest beds of the San Fernando sec- 

 tion were found three miles west of that town, and consist 

 of cross-bedded gray sandstones with a thickness of 60 feet. 

 Half a mile east the beds form a series of falls in the river 

 and we have the following section : 



Feet. 

 Conglomerate 4 



Cross-bedded sandstone indurated and with bands 

 of fossils 70 



Yellowish sandstones with fossils 3 



Gray sandstones, weathering in holes, few fossils 4 



The cross-bedded sandstone carries a great number of a 

 large Pecten, which are well preserved and, as it appears to be 

 a well marked horizon, we have called it the Pecten bed. It 

 is immediately overlain by beds of sandy clay with fragments 



