W J MCGEE — THE APrOMATTOX FORMATIOX. D 



few hundred square miles in the extreme northeastern corner, mantled western Ten- 

 nessee to and commonly a score or more miles beyond the Tennessee river, covered 

 western Kentucky nearly or quite to the Cumberland river, and extended across the 

 Ohio fifty miles or more into southern Illinois; and the isolated remnants west of the 

 Mississippi on the White, Arkansas and Washita rivers indicate a still wider exten- 

 sion westward. But, as in the territory previously studied, it has been extensively 

 invaded by erosion, cut through along the larger waterways and trenched by the 

 smaller, to such an extent that somewhere between thirty and seventy per cent, (and 

 probably between forty and sixty per cent.) of its original volume was carried into 

 the Gulf. In the axis of the Appomattox estuary, however, the formation was some- 

 times buried beneath newer deposits, and remains intact to afford artesian waters, as 

 at Memphis, Greenville and other points. The hypsographic distribution of the 

 formation in the Mississippi embayment ranges from 50 feet or less above tide on 

 Thompson bayou (near Bayou Sara) to 500-600 feet over the plateau extending north- 

 ward from Grand Junction, Tennessee, and to over seven hundred feet in the isolated 

 outliers between the Tennessee and Cumberland rivers about Tennessee Ridge. The 

 observations concerning stratigraphic relation and the inferences as to age made during 

 previous years are sustained and corroborated b}^ the season's work. The formation 

 rests unconformably alike upon the (probably Miocene) Grand Gulf deposits and upon 

 the Eocene and Cretaceous strata of the coastal zone, as well as upon the margin of 

 the Paleozoic terrane in Tennessee, Kentucky, Illinois and Arkansas. The distinc- 

 tive materials found in the Mississippi embayment only reflect the. rock composition 

 along the upper river and its tributaries in the usual manner; the enormous increase 

 in thickness is no more than proportional to the vast volume of the great river ; and 

 nearly all of the extensive erosion by which it is characterized occurred anterior to 

 the deposition of the Columbia formation, as is the case further eastward. 



The deposits of Mississippi now referred to the Appomattox formation were first 

 described by Professor Eugene W. Hilgard, the able expounder of southern geology 

 whose comprehensive grasp of isolated details and clear insight into complex rela- 

 tions excite the wonder and admiration of his followers, and whose interpretation of 

 a most obscure record was so complete that later students have modified his reading 

 only in minor points ; and the same series of deposits in western Tennessee was 

 described even earlier by another eminent pioneer among American geologists. Pro- 

 fessor James M. Safford. 



The results c^f the season's work raise a question of nomenclature. In 1856 the red 

 and orange sands of western Tennessee, now correlated with the Appomattox forma- 

 tion, together with certain other deposits, were described by Safford, designated the 

 " Orange Sand group," and referred to the Cretaceous.* In 1860 Hilgard adopted the 

 same designation for the extensive sand and gravel deposits of Mississippi, including 

 those now set apart and correlated with the Appomattox of Vir2:inia together with 

 some others, and referred the whole to the Quaternary. f Subsequently Safford modi- 

 fied the definition of his original group by excluding its lower portion, and at the same 

 time substituted the name " La Grange group " for the series thus defined, pointed out 

 its distinctness from the " Orange Sand'" of Hilgard, and referred it doubtfully to the 

 Eocene. J Loughridgehas recently adopted Safford's later designation for the greater 

 part of the same series as developed in western Kentucky (excluding '< the superficial 



* Geological Reconnoissance of Tennessee, pp. 148, 162. 

 t Geology and Agriculture of iMississippi, pp. 3, 4. 

 X Geology of Tennessee, 1869, pp. 150, 166, 424. 



