490 E. T. DUMBLE 



They are well shown in the valley of the San Juan and are 

 traceable for twenty-five miles south of that stream, when they 

 are covered by the Yegua. They are not observed on the Conchos. 



Fossils are abundant throughout the entire extent of the sub- 

 stage, although the upper portion is usually the most fossihferous 

 portion. In the Texas area in addition to the characteristic fauna 

 we find a few forms practically identical with those of the Tejon. 

 In the San Juan region, associated with the Venericardia planicosta, 

 there are found vast numbers of a varietal form very similar to, 

 if not identical with, V. potopacoenses of the Maryland Eocene. 



The succeeding substage, the Yegua clays, appears to be uncon- 

 formable with the Marine. The Yegua deposits are predomi- 

 natingly lignitic clays, but comprise beds of sands and greensands 

 as well. They, unlike thg earlier Lignitic, maintain their thickness 

 of 1,000-1,400 feet and carry their deposits of lignite entirely 

 across the state. They are also the principal gas-bearing beds of 

 the Eocene. Like the lower Lignitic, they become more sandy on 

 the Rio Grande and this condition persists throughout exposures 

 south of the river to Mendez on the Conchos or Presas. 



In their eastern part these beds are not as fossihferous as they 

 are west of the Guadalupe, although they carry fossils in many 

 places, especially between the Brazos and the Colorado. On the 

 Rio Grande they are nearly as fossihferous as the Marine. 



It has been suggested that the so-caUed Cocksfield Ferry beds 

 on the Sabine might be of Upper Claiborne age, but they have been 

 connected by careful tracing with the basal Yegua and are, there- 

 fore. Lower Claiborne. 



To these deposits of Yegua clay there succeeded another sub- 

 stage, the Fayette, which, while it carries beds of lignitic clays and 

 sands with some lignite, is principally composed of sands and 

 sandstones. Some of the sands are friable while others are qUartz- 

 itic and they carry a considerable amount of fossil wood which is 

 opalized instead of being simply silicified. They have a thickness 

 of 500-600 feet. 



East of the Guadalupe, with rare exception, the Fayette does 

 not carry any fossils except the wood, but west of that stream it 

 contains the large oyster, 0. alabamiensis var. contracta of Conrad, 



