THE PLIOCENE PERIOD. 303 



Lafayette, probably correspond to but a limited part of the complex 

 formation whose elements are many and intricate. On the west side 

 of the Appalachians, the formation seems to be essentially continu- 

 ous in the Tennessee valley as far north as Knoxville at least. 



The base on which the Lafayette formation rests is of slight relief, 

 and 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. In addition to the relief deter- 

 mined by erosion, the surface had a gentle slope to seaward. 



Thickness. — Like most sedimentary formations, the Lafayette is 

 variable in thickness. In general, it thickens seaward, and thins in 

 the opposite direction; 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 thickness 

 ranges from nothing to 200 feet or more. Sections of 20 or 30 feet 

 are common, and thicknesses greater than 50 feet are rare. 



Constitution. — The Lafayette is a very heterogeneous formation, 

 composed of gravel (and occasionally bowlders two or three feet in 

 diameter), sand, silt, and clay, variously related to one another. It 

 may be said to be both heterogeneous and homogeneous; that is, 

 there is considerable variation in its material within short dis- 

 tances, and but little more in great ones. In the lower Mississippi 

 valley, where the formation first attracted serious attention, and whence 

 the name is derived (Lafayette County, Miss.), it is predominantly of 

 sand and gravel, the coarser phases along drainage lines. In these tracts 

 it has usually the distinctive characteristics of fluvial sands and 

 gravels. The formation assumes a different phase over a broad tract 

 of the uplands east of the Mississippi and away from valleys generally. 

 In such situations it is composed largely of silt and clay. Some of 

 the clay is of exceedingly fine texture, and from such clay there are 

 various gradations into silt and sand. The formation is largely com- 

 posed of the insoluble residue of the older formations farther up the 

 slope on which the mantle lies, chert and quartz pebbles making up 

 the gravels, and other insoluble matter the fine constituents. 1 These 

 constituents replace one another at short intervals and in various 

 ways, and no systematic succession is observable. Lens-like masses 



1 Hilgard long ago pointed out (Am. Jour. Sci., Vol. IV, p. 266, 1872) that the 

 formation contains almost nothing which can be oxidized or readily dissolved. 



