126 W. J. McGee — Three Formations of 



gists, that acknowledgments are due to them for pertinent and 

 valuable suggestions, and that all coincide in the essential con- 

 clusions as to its genesis and age set forth in subsequent pages. 



The Potomac Formation. 



Character and Distribution. — The southernmost observed 

 occurrence of the formation is at Weldon, 1ST. C, where a bed of 

 obscurely stratified arkose, interspersed with well rounded 

 quartzite pebbles, appears in the north bank of the .Roanoke 

 beneath the railway bridge. The deposit rests on an unequally 

 eroded surface of gneiss, is not over a foot thick, and is unfos- 

 silif erous ; but its composition and structure are characteristic, 

 and there is little doubt as to its identity. 



Better exposures occur on the Nottoway river just below 

 Boiling's bridge. An irregularly and obscurely stratified 

 arkose, unquestionably belonging to this formation, here rises 

 four or five feet above low water, and is unconformably over- 

 lain by three or four feet of stratified greenish-blue clay contain- 

 ing Eocene fossils ; and exposures of similar character occasion- 

 ally occur in the river channel within the next five miles up 

 stream. In all of these outcrops the deposit exhibits a typi- 

 cal and easily recognizable aspect : it is a nearly homogeneous, 

 granular, loosely aggregated, almost mealy mass of quartz 

 grains (generally angular but sometimes rounded), flakes and 

 grains of kaolin (sometimes so little decomposed as to retain the 

 form of the parent crystals of feldspar), and scales of mica, 

 with an unimportant admixture of loamy particles, and now 

 and then a rounded or irregular pellet of white plastic clay. 

 The fresh surface is commonly light gray in color, but is fre- 

 quently flecked with white and stained with brown in curiously 

 curved lines. In structure it is either massive or obscurely 

 stratified and cross-laminated ; but even where the bedding is 

 most distinct it is rarely consistent and continuous for more 

 than a few feet vertically or a few yards horizontally. Except 

 for its obscure structure planes, its pellets and lenticular pock- 

 ets of clay, and the slight admixture of loamy matter, the mass 

 could hardly be distinguished from decomposed granite or 

 gneiss in situ. Some of these deposits on the Nottoway 

 river were observed and referred to the " secondary class of 

 rocks" by W. B. Rogers in 1839 ;* but a jprojpos to the inter- 

 esting question of geographic changes during the historic 

 period, it is significant that Eogers failed to find the formation 

 where it is now best exposed but found it " nearly on a level 

 with the water " at a point " about 4 miles above Boiling's 

 bridge " where it now rises fully 8 feet above the river ; the 



* Geology of the Virginias, 1884, 261. 



