132 W. J. McGee — Three Formations of 



as in the neighborhood of Broad Axe, near Three Tuns, three 

 miles northeast of Hatboro, and indeed generally on the sub- 

 ordinate divides and eminences northwest of Chestnut Hill and 

 Edge Hil],. The gravel in these outliers is fine (seldom over an 

 inch in diameter), well rounded, generally of quartzite, and 

 commonly imbedded in a scant matrix of arkose. A compact 

 pinkish quartzite is abundant and gives distinction to the 

 gravel. 



The substantial continuity of the formation as represented 

 by outliers is here broken, no exposures being known between 

 the east-flowing tributaries of Neshamin creek (near Church- 

 ville) and the Delaware river, a distance of nearly twenty miles ; 

 but two miles above Trenton on the New Jersey side of the 

 Delaware there is a distinctive deposit locally known as the 

 " Yellow Rocks," made up of arkose, sometimes lithified but 

 generally friable, containing abundant well rounded (but fre- 

 quently disintegrated) quartzite pebbles, disseminated, ar- 

 ranged in lines, or accumulated in pockets, together with 

 pellets and flakes of white plastic clay. The deposit is irregu- 

 larly stratified and inclines northwestward, but at a consider- 

 ably less angle than the immediately subjacent Triassic strata. 

 No fossils were found in the beds, but they are petrographi- 

 cally similar to those everywhere characteristic of the lower 

 division of the Potomac formation, totally unlike those of the 

 Triassic sandstones in structure, composition, and attitude, 

 equally unlike the Raritan clays found in the vicinity, and 

 quite distinct from the gravels and loams of the Columbia 

 formation by which they are overlain ; and on the whole it 

 seems evident that the deposit represents the sandstone mem- 

 ber of th.e Potomac formation. Somewhat similar and prob- 

 ably contemporaneous deposits of stratified sand occur two 

 miles below Trenton beneath the Raritan clays. 



Midway between Princeton and New Brunswick, N. J., an 

 anomalous and hitherto unclassified deposit of friable ferrugin- 

 ous sandstone crowns the southernmost of the Triassic trap 

 ridges, and is locally known as the " Sand Hills."* It con- 

 sists of massive or irregularly stratified and sometimes cross 

 laminated brown sands, occasionally cemented by ferruginous 

 infiltration, with a few intercalated lines of white or pinkish 

 plastic clay, and is strikingly similar to the non-kaolinic sands 

 of the Media outlier and of the body of the formation as ex- 

 posed north of the head of Chesapeake Bay. This outlier is 

 completely isolated and unfossiliferous ; but it is distinct from 

 the Triassic sandstone on both sides of the ridge in all diag- 

 nostic features, and is, moreover, manifestly newer than the 

 trap dike, which was itself formed after the sandstone was de- 



* Geology of N. J., 1868, 227, 342. 



