W J MCGEE — THE APPOMATTOX FORMATION. 6 



istics as to be readily recognized wherever seen. It occupies the greater part of 

 the coastal plain in the southern Atlantic and eastern Gulf slopes, and overlies un- 

 conformably the Miocene, Eocene and Cretaceous formations ; and it is in turn uncon- 

 formably overlain by the Columbia formation. In its typical development within 

 this area the deposit is frequently flecked and streaked with white ; and it becomes 

 gravelly toward waterways, and also differentiates into stratified beds of sand and 

 clay toward the coast. The pebbles in the gravelly portions are of rocks forming the 

 terranes traversed by the rivers along whose lower courses they are accumulated, and 

 so difler from river to river ; the bulk of the loam is evidently composed of residuary 

 sands and clays such as exist in greater or less volume along the same rivers ; and the 

 white material by which the deposit is flecked and streaked is in considerable part a 

 kaolin or clay apparently resulting from decomposition of feldspar, which forms an 

 element in the Piedmont crystallines overlapped by the coastal plain formations. 

 , During the present season the formation was studied in some detail in western, cen- 

 tral and northern Mississippi, and in western Tennessee and Kentucky ; and it was 

 also recognized and casually studied in southern Illinois, as well as on the other flank 

 of the Mississippi embayment in central Arkansas. Within this region the formation 

 was found to display certain peculiarities in composition and structure which vary 

 from place to place in a significant way, and also to exhibit a notable increase in 

 volume. 



In northern Louisiana and the contiguous portion of Mississippi the formation is a 

 loam similar to that of southeastern Mississippi and Alabama, save that the prevail- 

 ing color is brick-red rather than orange and that it is frequently pebbly, the pebbles 

 consisting mainly of subangular and rounded fragments of Paleozoic chert. The 

 thickness of the formation here was not accurately determined, but is apparently less 

 than that attained further northward ; and as usual it has been profoundly eroded. 

 It is commonly overlain by the Columbia formation up to altitudes of 250 feet or 

 more. In central Mississippi the Appomattox loams and sands commonly form the 

 surface over a considerable zone parallel with the Mississippi, but dip beneath the 

 Columbia deposits toward the bluffs of that river and along its larger tributaries. 

 Here the deposit is thicker, more sandy, and more frequently gravelly than to the 

 southward ; and moreover it displays a differentiation of materials in the vertical 

 direction : the upper third is a fairly homogeneous brick-red or orange sandy loam, 

 sometimes gravelly ; the middle third a similar loam or sand of like color interstrati- 

 fied with thin lenses of white silicious clay; while the lower third is an irregularly 

 stratified bed of silt, clay and sand. Exposures of 40 to 60 feet are not uncommon, 

 and the full thickness of the formation probably exceeds a hundred feet; but as usual 

 it is deeply dissected and sometimes completely removed along the waterways. In 

 northern Mississippi and in western Tennessee and Kentucky the vertical diff'erentia- 

 tion commencing in the first-named state is well defined, and the formation becomes 

 a definitely tripartite one. (1) The upper member is a fairly uniform brick-red sandy 

 loam locally charged with pebbles, perhaps in such number as to transform the mass 

 into a gravel bed ; sometimes it is cross-stratified, and it may be flecked and streaked 

 with white, as in the more easterly localities ; and, as in these localities, exposed surfaces 

 frequently display the distinctive semi-glazed aspect resulting from alteration of the 

 contained iron. Over the terranes of the more ferruginous Eocene formations this 

 member, and indeed the entire formation, is exceptionally rich in iron ; the gravels are 

 sometimes so firmly cemented as to form puddingstones, the sands are locally lithified, 

 and plates and nodules of sand ironstone are common. (2) The middle member may 

 be a nearly pure snow-white silicious clay, but more commonly consists of alternating 



