1 16 GEOLOGY. 



The Trinity series, 1 the oldest member of the system in Texas, 

 is unconformable on the Triassic or older rocks. Its fossils are such 

 as to have raised the question of its reference to the Jurassic system, 

 but it is not commonly so classified. The basal part of the formation 

 is like the Potomac of the east, in being non-marine, but the upper 

 parts were deposited in sea- water. The series consists of sands, clays, 

 marls, and limestones. In the lower part of the series any one of 

 these various sorts of rock grades into any other, vertically or 



Fig. 381. — Section showing position of the Comanchean beds near Austin, Texas. The 

 amount of faulting is exceptional. Length of section about 4 miles. (U. S Geol 

 Surv.) 



horizontally. 2 The series contains both asphalt and bitumen. 3 It 

 extends northward to the Ouachita Mountains in Arkansas and Indian 

 Territory, where the waters of the epoch appear to have had their 

 shore. After the deposition of more than 2000 feet (maximum) of 

 sediment, there appears to have been a shoaling of the waters, followed 

 by a deepening which inaugurated the next epoch. 



The Fredericksburg series, which overlies the Trinity, is more 

 widespread than its predecessor, though it does not now cover all of 

 the former, because of subsequent erosion. The series extends north 

 to the Ouachita uplift, and perhaps around its western end over 

 a limited area farther north, and west to New Mexico. The earliest 

 beds of the series are clastic, and of shallow- water origin; but thick 

 beds of limestone (or chalk) occur in other parts of the series. In the 

 vicinity of the shores, especially next to the Ouachita uplift, where 

 the shore phases of the formation are best known, the formation is 

 relatively thin and mainly clastic. The Fredericksburg series is 

 much less variable, both in thickness and composition, than the Trinity 

 series below, and contains more calcareous material. 



The Fredericksburg formation is overlain by the Washita, a series 

 which records an epoch of shoaling waters, though the sea was some- 



1 Hill, op. cit., p. 129 et seq. 



2 Idem. 



3 Eldridge, Bull. 213 U. S. Cxeol. Surv., p. 301. 



