262 



PROCEEDINGS OF THE ACADEMY OF 



[1880. 



two or three feet deep. The boulders of the Nicetown clay are 

 similar to those of the Neck clay, except in the fact that in the 

 latter there are numerous rounded and sharp fragments of triassic 

 red shale, while in the former boulders of that material are very 

 scarce. The boulders of both clays are invariably derived from a 

 northern source, ^o shells or organic remains have as 3'et been 

 noticed in this formation. 



Beneath the clay, and often unconformable with it, is the Phila- 

 delphia red gravel. It is a claye^'^ gravel which packs well and is 

 much used on roads, and whose red color is caused by the ferru- 

 ginous clay in which the pebbles are imbedded. The pebbles are 

 composed of all kinds of rock and are not flattened as are those in 

 the river gravel. The predominant material is white quartz, but 

 pebbles of all other materials, as conglomerate, sandstone, fossil- 

 iferous hornstone, flint, red shale, etc., are numerous. Stratifica- 

 tion is observed in almost every section exposed. Good sections 

 of gravel are seen near the University of Pennsylvania. It has 

 here an elevation of about 50 feet, and comes to the surface of the 

 ground with but a very slight covering of clay. The gravel is 

 here over 15 feet deep, and as it is in some respects a typical 

 exposure, a section is herewith presented. 

 Ft. Fig. 1. 



10 



1 



5 



3 

 1 



+ 



BRICK-CLAY. 



RED GRAVEL. 



BLACK GRAVEL. 



YELLOW GRAVEL. 



MICACEOUS SAND. 



BOULDERS. 



DECOMPOSED GNEISS. 



