THE PLIOCENE PERIOD 831 



the Mississippi basin, and is represented, if our interpretation is 

 correct, (4) in the valleys west of the Appalachians. 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 usually applied only to the formation on the slope be- 

 tween the Appalachians and the Atlantic, about the Gulf, and in 

 the Mississippi basin below the Ohio. Thus limited, the formation 

 has been estimated to have an area of some 250,000 square miles. 

 It lies upon the eroded edges of all older formations of the region, 

 from pre-Cambrian to Miocene. It extends inland from the coast up 

 to altitudes of 1,000 feet l near the Rio Grande, 800 feet in Tennes- 

 see, and 300 to 500 feet on the Atlantic slope. 



At its mountain-ward edge, ragged belts of the Lafayette forma- 

 tion follow the valleys up into the mountains, and unless our iden- 

 tifications are in error, they reach back with local interruptions, 

 into the intermontane valleys. 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 now known on land is demonstrably marine. 



It is not to be understood that the Lafayette formation occurs 

 everywhere within the general area of its distribution. Over con- 

 siderable areas, it caps divides, but is absent from the valleys be- 

 tween them, obviously the result of stream erosion. The base on 

 which the formation rests has but little relief, and a gentle dip sea- 

 ward. It appears to have been either in an advanced stage of 

 erosion when the Lafayette formation was deposited, or too low to 

 have become notably rough as a result of erosion. 



Thickness. In general, the formation thickens seaward; but 

 at any given distance from the sea, it is thicker in the valleys which 

 affected the surface on which it was deposited, and thinner on the 

 divides between them. The known thickness ranges from to 200 

 feet or more. Sections of 20 or 30 feet are common, and of more 

 than 50 feet are rare. 



Constitution. The Lafayette is composed of gravel (and occa- 

 sionally bowlders), sand, silt, and clay, variously related to one 



1 McGee, loc. cit. 



