THE PATUXEXT FORMATION. 63 



The total exposed thickness of the Potomac formations in Virginia in 

 the region of their outcrop is about 500 feet. The formations thicken down 

 the dip to the eastward, and in all probability the Earitan also, although 

 undifferentiated, is present beneath the Tertiary cover. At Fortress Monroe 

 the combined thickness of the Potomac formations, penetrated in the new 

 government well, is about 1.300 feet. 



The Patuxent Formation. 



Nanif. — The Patuxent formation, so called from the Patuxent Eiver, 

 Maryland, where the deposits are well exposed, constitutes the basal portion 

 of the Potomac group. 



Stratigraphic relations. — The Patuxent beds rest throughout most of 

 their area, with marked unconformity, upon the crystallines of the Pied- 

 mont and the contact is observable at numerous outcrops. In a limited 

 area around Doswell the Patuxent rests upon the Triassic. In most of 

 the region from Alexandria to Fredericksburg the Patapsco unconformably 

 overlies it. South of Fredericksburg either the Eocene or the Miocene 

 deposits directly overlie the Patuxent. The Lafayette and Pleistocene 

 deposits also frequently cover the Patuxent outcrops along the larger 

 valleys. 



Litliologic character. — The materials composing the Patuxent are vari- 

 able, the formation frequently changing its lithologic characters rapidly in 

 both vertical and horizontal directions. The deposits consist chiefly of 

 light-colored sands, sometimes almost entirely made up of pure quartz 

 grains, but usually containing a large amount of kaolinized feldspar 

 (arkose) resulting from the decomposition of granitic rocks in the region 

 to the westward. Mica flakes derived from the same source are very 

 abundant at certain localities as are also greenish clays which derive their 

 color from the chloritic schists of the Piedmont. Above Dutch Gap along 

 the James River certain beds composed of quartz, mica, and kaolin so 

 closely resemble the residual materials resulting from the decay of the 

 granitic rocks that the planes of stratification are almost the only clue to 

 their real origin. The quartz grains are generally quite well rounded, 

 although in some places, as in the locality above cited, they are decidedly 

 angular. The .sands locally become indurated, sometimes so firmly as to 

 he of value as a building stone. Before the day of railroads these indurated 

 layers were quarried in a number of places. At Aquia Creek the United 

 States government in the early part of the last century quarried a sandstone 

 from this formation for the construction of the White House, the old por- 



