486 E. T. DUMBLE 



These conditions continue to the Salado River, but south of 

 that stream the Midway covers a much larger surficial area and the 

 Wilcox and Carrizo are less prominent. 



By far the best exposures of the Midway which we have found 

 are those in the valley of the Salinas River east of Ramones, 

 Mexico. Here they occupy a considerable area and also form 

 the Alto Colorado, a hill 250 feet in height which is composed 

 entirely of Midway sediments. These have a total thickness of 

 more than 400 feet. Among the fossils found in some abundance 

 are Ostrea pulaskensis, O. crenulimargo, Venericardia alticosta, 

 V. planicosta. 



The last exposure of these beds within this area was found on 

 the road to China, six miles southeast of Ramones. 



That the close of the Midway was brought about or accom- 

 panied by an uplift is proven in the Rio Grande region, where it 

 is seen to have a greater dip than the succeeding Lignitic series. 



Wilcox or Lignitic series. — The series of fresh- water or estuarine 

 deposits which followed the Midway shows little change in the 

 general character of its sediments southwestward, with the excep- 

 tion that south of the Colorado it loses the larger part of the 

 lignitic beds which are such a prominent feature of it north and 

 east of that river. 



North and east of the Colorado the Wilcox covers a broad area 

 and has a thickness, as shown by wells drilled through it, of 1,000- 

 1,200 feet. South of this river the exposure gradually becomes 

 narrower and the beds thinner. 



Its exposures along the Rio Grande are few and the thickness 

 of the beds exposed probably does not exceed 100 feet. All of its 

 observed contacts with the Midway were unconformable and there 

 is a distinct difference in the degree of dip of the beds of the two 

 stages. The sand and clays of the Wilcox Lignitic show evidence 

 of emergence and erosion and in many places are entirely absent, 

 having evidently been eroded prior to the deposition of the lowest 

 Claiborne sands. 



The Lignitic is found within a few miles south of the Rio 

 Grande in a number of exposures. Farther south and beyond 

 the Salinas it overlaps and covers the Midway, so that the latter 



