600 THE PLIOCENE PERIOD 



have been developed since their deposition, for their remnants are 

 on the highest lands within the area where they occur. 



The Lafayette formation. 1 About the Atlantic and Gulf coasts 

 similar deposition gave rise to the Lafayette (Orange Sand) forma- 

 tion, which seems to have had a history somewhat like that of 

 the Pliocene beds of the west, though this interpretation has been 

 challenged. This formation has an extensive distribution (i) 

 between the Piedmont plateau and the Atlantic, (2) on the inland 

 part of the Coastal Plain of the Gulf of Mexico, and (3) in the south- 

 ern part of the Mississippi basin, and is represented, if our inter- 

 pretation is correct, (4) in some of the valleys of the Appalachians 

 and west of them. On the Coastal Plain of Texas the formation 

 is connected with analogous deposits on the Great Plains, and 

 through them with the intermontane deposits of the west, already 

 mentioned. The term Lafayette has been applied only to the 

 formation on the slope between the Appalachians and the Atlantic, 

 about the Gulf, and in the Mississippi basin below the Ohio, where 

 it lies upon the eroded edges of older formations, and extends in- 

 land from the coast up to altitudes of 1,000 feet 2 near the Rio 

 Grande, 800 feet in Tennessee, and 300 to 500 feet on the Atlantic 

 slope. At its mountainward edge, ragged belts of the Lafayette 

 formation follow the valleys up into the mountains. At its seaward 

 margin, it is more or less completely concealed by younger beds, and 

 it is not to be doubted that it passes out to sea beneath them. No 

 part of the formation on land is demonstrably marine. 



Within the general area of its distribution the formation is not 

 continuous. Over considerable areas, it caps divides, but is absent 

 from the valleys between them, obviously the result of stream ero- 

 sion. The base on which the formation rests has but little relief, 

 and a gentle dip seaward. 



In general, the formation thickens seaward. Its known thick- 

 ness ranges from o to 200 feet or more, sections of 20 or 30 feet 

 being common. 



1 The fullest sketch of this formation as a whole is that of McGee in the Twelfth 

 Annual Report of the U. S. Geological Survey. A few references to other accounts 

 of the formation in special localities, some of them under other names, are as follows : 

 Safford, Am. Jour. Sci., Vol. XXXVII, 1864; Hilgard, Agric. and Geol. of Miss., 

 1860, and Am. Jour. Sci., Vol. XLI, 1866, and Vol. IV, 1872; Salisbury, Geol. 

 Surv. of Ark., Report on Crowley's Ridge, 1889; Durable, Jour. Geol., Vol. II, 

 1894, p. 560; Smith, E. A., and Johnson, L. C., Geol. Surv. of Ala., 1894. 



2 McGee, loc. cit. 



