128 W. J. McGee — Three Formations of 



and diked, Rhsetic beds. Here too, the quartzite pebbles, so 

 abundant on the James and Appomattox, disappear. 



About Fredericksburg the formation is well exposed and ex- 

 hibits some noteworthy characters : it consists predominantly 

 of locally lithified arkose with abundant pebbles either irregu- 

 larly disseminated or arranged in bands and beds, and numerous 

 bowlders of gneiss and vein-quartz (quartzite being altogether 

 absent) up to two feet or more in diameter, together with hete- 

 rogeneous sand containing a considerable element of finely 

 divided diffused clay ; while lenticular beds of clay are fre- 

 quently intercalated in both arkose and sand, and in some expo- 

 sures constitute the major part of the mass. As on the James, 

 the clay is massive, obscurely bedded, or finely laminated, and 

 sometimes contains abundant leaf impressions ; and here, as 

 elsewhere, silicified and lignitized wood are common in the sand 

 and clay respectively. 



Thus, south of the Rappahannock the formation consists of 

 an indivisible mass of arkose, sand, clay and gravel, generally 

 overlapped by the Tertiaries, and exposed only along waterways 

 and over a limited area between the two Anna rivers. 



INorth of the Rappahannock the formation increases in vol- 

 ume and in diversity, and forms the surface over a zone several 

 miles in width between the Eocene margin and the Piedmont 

 crystallines. Near the mouth of Acquia creek it has an observed 

 thickness of 180 feet, mainly of lithified arkose (long known 

 as a fairly valuable building stone and extensively used in old 

 structures in Washington, Alexandria, and Fredericksburg), 

 intercalated with plant-bearing clay beds. Midway between 

 Fredericksburg and Acquia creek the characteristic phases of 

 the formation as developed in the south are found to be over- 

 lain by a bed of clay of obscure or inconstant structure, con- 

 taining occasional intercalations of arkose or beds of pebbles ; 

 and this clay deposit, which lies beneath the Eocene and is 

 generally unfossiliferous, increases in thickness northward. At 

 the same time, isolated patches of gravel identical with that of 

 the body of the formation, and evidently outliers insulated by 

 erosion, begin to appear along the marginal portion of the 

 Piedmont zone, generally capping eminences of circumdenuda- 

 tion. These outliers are sometimes 25 or 30 miles within the 

 fall-line. 



Between Acquia creek and Washington the formation con- 

 tinues to form the surface over a considerable zone ; its lower 

 part maintains the characters exhibited farther southward, as 

 shown by excellent outcrops at many points on the Potomac 

 river, and in ravines and railway cuttings to the westward ; and 

 the superjacent clays progressively increase in thickness with- 

 out material change in character. On Occoquan river quartzite 



