190 PROCEEDINGS OF THE ACADEMY OF [1890. 



tobluij-h in color. In the northwest corner of the basement this bed 

 was found ten feet below the pavement ; from this point it has a slight 

 but irregular dip toward the southeast. It has a thickness of about 

 six feet and rests upon gravel which varies in different ])laces from 

 a sharp sand to a coarse gravel containing some extra large pebbles. 

 Enclosed within the clay on the 9th street side are some thin len- 

 ticular-shaped deposits of sand. 



A careful examination of all portions of the clay which was here 

 exposed over an area of 120 feet by 132 feet revealed the presence of 

 .sponge spicules in considerable numbers, Init more numerous at the 

 bottom than at the top of the bed. Many were of the pin-head 

 forms characteristic of salt water. A very few marine diatoms 

 Avere also observed. 



Within a week after the preceding investigations some bluish 

 clay belonging to the same bed was obtained from an excavation at 

 9th & Race streets. The clay is here a few feet nearer the street 

 level. At this point sponge spicules and diatoms occur, both in 

 abundance. The diatoms show a mixture of salt water and fresh 

 water forms. About fifty species have been observed, two-thirds 

 being marine and one-third fresh water. Numerically, how- 

 ever, probably seven-eighths of the individual forms are marine. 

 Among the sponge spicules the characteristic marine forms with pin 

 head termination at one end were here again observed. Here also 

 the life forms were more plentiful in the lower part of the bed. 



What is probably the same clay bed and likewise containing 

 marine spicules and diatoms has been met with at 11th and Cherry 

 streets, about two feet below the pavement in an excavation for an 

 underground telegraph conduit. 



Clay with marine spicules has also been obtained from a depth of 

 two and a half feet at 15tli and Summer streets and also from be- 

 neath the basement floor of the City Hall. 



The inference to be drawn from these facts is that the former and 

 wider ocean estuary of Delaware Bay covered the site of Philadelphia 

 with its saline waters and that its life forms were mainly marine, but 

 that it w^as subject to considerable invasion of fresh water from tlie 

 river and neighboring streams, and thus fresh water forms were 

 brought into association with marine species. 



Mixtures of salt and fresh-water diatoms have also been found in 

 deposits near the present borders of the Delaware and Schuylkill 

 rivers. They occur at lower levels than that of the clay bed at 9th 

 and 3Iarket streets and probably belong to a lower flood plain of the 

 rivers and would therefore be more recent in date. The Delaware 

 Bay had by this time considerably diminished in width but the pres- 

 ence of marine diatoms would indicate the continuance of ocean water 

 influences up to the latitude of Philadel[)hia until quite recently. 

 The assemblage of salt and fresh water diatoms now referred to were 

 found in borings preliminary to work done or to be done for the 

 foundations of the eastern abutments of the Chestnut and Walnut 



