STRATIGRAPHY. 15 



•in Northern Texas the distance across its outcrop is over thirty miles, east of 

 Austin the distance is less than fifteen miles; yet in both localities the dip is 

 about the same. Of course these differences in the thickness of the bed may 

 be partly due to a lesser deposition of the " Ponderosa Marls " in the latitude of 

 Austin than in the northern part of the State, or else to the greater overlap- 

 ping of the Tertiary strata, yet the character of the country is such as to prove 

 that a great amount of erosion has taken place. The Tertiary deposits of East 

 Texas, overlying these Cretaceous strata, consist of a vast thickness of sand, 

 clay, and glauconite beds, in some places characterized by great quantities of 

 lignite, and in others by beds of littoral fossils. In fact the whole series 

 represents a succession of coastal, subcoastal, or brackish water* deposits, al- 

 ternating with marine deposits of a littoral character, and between these two 

 extremes we find all gradations. The lagoon or subcoastal deposits compose 

 by far the greater part of the series, and the marine strata represent slight 

 and temporary submergences of the coastal area. The proofs of the littoral 

 character of the marine deposits are many, and may be summed up as fol- 

 lows: 



1. The fossils all represent a littoral marine fauna. 



2. Fragments of lignite are frequently found in the marine beds. These 



must have come from the destruction of a lignite bed on the shore, and 

 have been carried into the sea by rivers tributary to it. They must have 

 been deposited near the shore, as such soft fragments would have been 

 rapidly broken up before they got far out to sea. A bed of this kind is 

 seen at " Bombshell Bluff" on the Colorado River, where numerous frag- 

 ments of lignite are mixed in with glauconitic beds containing many 

 marine fossils. 



3. Frequently lumps of clay or sandy clay are embedded in strata bearing 



marine shells. These must have been deposited near the coast line, as 

 they could not have stood long transportation without disintegration. 

 A remarkable instance of this is seen in a bluff on the Brazos River at 

 the crossing of the International and Great Northern Railroad, where 

 there occurs a bed of glauconite underlaid by lignitic clays. Fragments 

 of this clay, one to twelve inches in diameter, are found in various parts 

 of the overlying bed. 



4. The marine beds are not always continuous, but blend laterally into beds 



of brackish or fresh water origin. Glauconitic beds containing fossils in 

 one place are often represented in another by beds of pure siliceous sand 

 or by clays containing large amounts of vegetable matter. Such an effect 



*This same effect might also be brought about in salt water bays and estuaries into which 

 the rivers from the inland flowed. 



