Vol. V] DUMBLE— TERTIARY DEPOSITS /.V NORTHEASTERN MEXICO IgZ 



South of the Sah'nas River on the road from China to 

 Laguna de los Indios we find, beginning a mile west of 

 Chilarios, a series of yellow clays with nodules of red clay, 

 ironstone and gypsum, which apparently belong to the Frio. 

 Southeast of Chilarios all the washes and gullies show the 

 vellow gypseous clays with only a few shaly sandstone beds. 

 These carry the small oyster of the Frio. Just how far these 

 beds extend to the southeast before they are covered by the 

 yellow sandy clays of the Oligocene, could not be determined 

 owing to lack of suitable exposures. Similar clays were, 

 however, observed 24 miles southeast of Chilarios. The best 

 development of these beds was found in the region of the 

 Conchos River. Between Tepetate and the San Francisco 

 Ranch, northwest of San Fernando, they comprise yellow 

 clays and soft gray sandstones, dipping northeast and carry- 

 ing oysters. The hills stretching northwest from this locality 

 are largely made up of these clays and sands with beds of 

 gypsum and in the Sierra de Pomeranes they also show a 

 considerable thickness and are interbedded with or carry 

 gypsum in all its varieties. Here they are underlain by the 

 Fayette and capped by the San Fernando. At San Diego, 

 which is at the southern point of these hills, the Frio shows 

 in a ridge capped with three feet of massive gypsum underlain 

 by greenish clays weathering white and carrying the Frio 

 oysters. Their extension south of the Conchos, if any, has 

 not yet been worked out. 



So far as our investigations show, all exposures of Lower 

 Eocene (Midway and Wilcox) deposits are confined to the 

 limits of the present drainage basin of the Rio Grande. 

 Whether this coincides with the limits of the Rio Grande em- 

 bayment of Lower Eocene time cannot be stated. The de- 

 posits of the Middle Eocene, however, extend south through 

 the basin of the Conchos River. 



From the evidence before us it appears that following the 

 close of the Wilcox deposition there was a period of elevation 

 and erosion, succeeded at the beginning of the Lower Clai- 

 borne by a rather rapid incursion of the sea which trans- 

 gressed the earlier Tertiary area in places and allowed the 

 deposition of the Carrizo sands. During the succeeding sub- 

 stages of the Lower Claiborne there was a gradual sinking 



