1880.] NATURAL SCIENCES OF PHILADELPHIA. 269 



all traces of Triassic red shale, a formation of large extent six miles 

 north of here, over which such a flood must have passed, is difficult 

 to explain upon that hypothesis. This belt of cla}', which may be 

 called for convenience the " Branchtown clay," extends S. W. 

 to Chelton Avenue and Chew Street, in Gerniantown, and to the 

 N. E. to Limekiln Pike and City Line Road, and is the site of 

 several brickyards. The clay plateau is bounded on the N. W. by 

 a hill 325 feet high. Doubtless this clay will be found in other 

 places, when more light will be thrown upon its origin and age. 



The Bryn Maior Gravel. — Upon the summits of some of the 

 highest hills in the gneissic region back of Philadelphia, at a mean 

 distance of about nine miles from the river, and at elevations of from 

 325 to 450 feet above it, there are isolated patches of an ancient 

 gravel, different from any yet described, to which we have given the 

 provisional name of " The Brj'n Mawr Gravel." It can always 

 be recognized b}- the presence of sharp or partially rounded frag- 

 ments of a hard, heavj^ iron sandstone or conglomerate. Such 

 fragments are often covered by a brownish-black iron glaze. More 

 than ten years ago, the writer noticed in the soil of the upper part of 

 Gerraantown, pieces of this conglomerate, unlike any known rock, 

 and it is only of late that its origin has been suspected. It con- 

 sists of well-rounded pebbles of quartzite or siliceous sandstone 

 cemented by iron into a stone which is often very hard. This 

 conglomerate is found in occasional fragments upon ground over 

 300 feet high, but is not found in abundance until an elevation of 

 over 400 feet is reached. At these highest points it occurs in a 

 red gravel wliose pebbles are identical with those of the conglom- 

 erate. 



One of such points is near Chestnut Hill, on the City Line 

 Road at its highest elevation, near Willow Grove Road. Here, 

 nearly nine miles from the river and 425 feet above it, is a patch 

 of this gravel and conglomerate. The larger pebbles and boulders, 

 like those of the Branchtown Cla}', consist of a friable quartzite 

 sandstone or a jaspery quartzite. Sharp fragments of quartzite 

 are numerous ; but there are no traces either of Triassic red shale, 

 of fossiliferous pebbles, or of rounded pebbles of the uiiderlying 

 gneiss. It rests upon a much decomposed gneiss. The conglom- 

 erate sometimes contains cavities filled with white sand. The 

 tract of gravel is of an oval form, whose major axis points N. E. 

 and S. W. It crosses the Township Line Road near the Bethesda 



