1880.] NATURAL SCIENCES OF PHILADELPHIA. 261 



the river is too stiff to 1)6 useful for brickmaking. Blackened 

 fragments of twigs, roots and leaves are frequent in it, and it is 

 said that trunks of the white cedar abound in it in some places. 

 There is here an indication that these beds are sinking and that, 

 as on the Atlantic coast, the water is encroaching. Frequently 

 a good peat covers the clay. 



The River Gravel. — Forming the Floodplain Terrace and lying 

 back of it, is a light sand and gravel free from claj', which may 

 be designated the " River Gravel," since it formed the ancient river 

 bed. It is composed of a light micaceous sand made from the 

 wear of gneissic rocks, ovcrl3'ing a clean, loose gravel, whose peb- 

 bles are composed of the rocks which form the river bottom farther 

 north. The pebbles are generalh' flattened and are composed of 

 gneiss, Triassic red shale, Triassic argillite, etc. It is of a gray 

 color, white quartz pebbles being comparatively^ scarce. It under- 

 lies the river to a great depth and forms islands in it. Frequently 

 large boulders lie upon the river gravel. Bridesburg and the 

 Lazaretto are built upon it. The sand is used for building pur- 

 poses. It is bounded by the " i?u-e?- Gravel Terrace," a terrace 

 rising some twenty feet above mean tide, and which is capped by 

 the red gravel and brick-clay about to be described, while rocks 

 are frequently exposed at its base. The Chester Branch of the 

 Reading Railroad lies below this terrace, and the present line of 

 the Philadelphia, Wilmington and Baltimore Railroad is above it. 



The Bed Gravel and Brick-Clays.— T\\q built-up portion of the 

 city stands upon an extensive deposit of brick-clay and gravel, 

 sections of which are exposed in ever}' cutting. The brick-clay 

 invariably overlies the gravel, and will therefore be first described. 

 By far the finest exposures of brick-clay are those on either side 

 of Long Lane, in the "Neck." The clay here is ver}" compact, 

 free from sand and gravel, and is often 15 feet or more in depth. 

 Loam lies above it, and is mixed with it for brick-making. Well- 

 rounded boulders of Potsdam, Medina, Trias, etc., are frequent. 

 The whole lies upon some 20 feet of stratified gravel. It is a 

 much finer and deeper clay than that of the northern part of the 

 city, as at Nicetown. It is interesting to note that while the clay 

 which is farthest from the Upland Terrace and lowest in elevation 

 is purest and deepest, on the other hand that near the terrace 

 and more than 100 feet above the river is both shallow and sand3\ 

 It suggests that the former was deposited in deep water and the 

 latter near the shore. At the base of the terrace the clay is but 



