832 GEOLOGY 



another. It may be said to be both heterogeneous and homoge- 

 neous; that is, there is considerable variation in composition in 

 short distances, and but little more in great ones. In the lower 

 Mississippi valley, whence the name is derived (Lafayette County, 

 Miss.) it is of sand and gravel chiefly, the coarser phases being along 

 drainage lines where it has the distinctive characteristics of fluvial 

 sand and gravel. Over a broad tract of the uplands east of the 

 Mississippi and away from valleys generally, it is composed largely 

 of silt and clay. Its constituents are largely the insoluble residues 

 of older formations farther up the slope on which the formation 

 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 system- 

 atic succession is observable. Lens-like masses are not uncommon. 

 Irregular stratification is the rule, but some portions are not bedded 

 or laminated. Certain lenses of sand suggest an eolian origin. 

 A singular pebble-earth that finds its analogue in subaerial and 

 flood-plain deposits is common. 



The color of the formation ranges from brick-red through various 

 pinks, purples, oranges, and yellows, to white. The color is more 

 irregular than the composition, bands, blotches, and mottlings 

 diversifying the structural units. 



Fossils. Fossils are rare. In its unquestioned and represent a- 

 tive parts they are all of land plants and animals (except, of course, 

 the fossils derived from earlier formations). The formation is so 

 much dissected that it is well exposed to observation, and the 

 rareness of known fossils is not due to lack of adequate search. 



Origin. The preferred interpretation of the Lafayette formation 

 is as follows : After the base-leveling of the region before the ( '< >- 

 manchean period, 2 the Appalachian tract was bowed up and a 

 new stage of degradation inaugurated. During the earlier part of 

 the Tertiary, a partial peneplaning of the less resistant tracts waa 

 accomplished, accompanied by relative subsidence along shore. 



1 Hilgard, Am. Jour. Sci., Vol. IV, p. :>r>(>, lX7i>. 



2 Rivers of Pennsylvania, and ( Ico^raphic Development of Northern \-\v 

 ey, Nat. Geo. Mag., Vol. I and Vol. II. respectively. Hayes, eliapter mi 



the southern Appalachians, in Physiography of the United States. 



