INTRODUCTION 



The Delaware River rises in the Western Catskill Mountains, flows southward for about 

 three hundred and seventy-five miles, and expands into Delaware Bay about sixty miles from 

 the sea. Its origin is among the Devonian and Carboniferous rocks, and in its course it passes 

 through Silurian, Triassic and Cretaceous formations, finally reaching the Cambrian and 

 Laurentian beds. It also drains regions of the glacial drift and beds which overlie over- 

 turned Miocene strata, and are sometimes mixed with them. From the mountains, nearly 

 four thousand feet high, to the Bay, where the depth of water is not greater than seventy- 

 five feet, the diatomaceous flora, from Alpine cascades to the salt marshes of New Jersey, 

 contains a larger number of species than any other equal portion of the American coast. 



The city of Philadelphia, about one hundred miles from the sea, lies at the junction of 

 the Schuylkill with the Delaware, and much of the land near the rivers, especially south- 

 ward, is flat and low, composed of recent alluvial deposits. In the central districts the 

 ground is high, the deep sub-soil being mostly a dry gravel resting upon gneiss and schist, 

 although it is in part composed of a bluish clay which was probably laid down in the bed of 

 the ancient river before the last period of the glacial drift. The blue clay was not all de- 

 posited at the same time, as in the lower strata many marine forms are found which do not 

 occur in the upper layers. This is notably the case in a deposit obtained at Spreckel's Sugar 

 Refinery and also at the east end of Walnut Street Bridge, where a layer of blue clay occurs 

 which is overlain by glacial drift. In other parts of the city mixtures of blue clay with more 

 recent deposits are found, including fresh-water forms from numerous creeks and rivulets 

 which traversed what is now the city proper, and especially from the vicinity of Fourth and 

 Market Streets, where there existed as late as the year 1700 a large pond known as the "Duck 

 Pond" which was subject to tidal overflow from its outlet, Dock Creek. The river water at 

 Philadelphia is not noticeably brackish, although the tide extends thirty miles above the city 

 and, before the building of Fairmount Dam, to the Falls of the Schuylkill. At certain times, 

 when the river is low, the influx of tide water is sufficient to produce an abundance of brack- 

 ish water diatoms at Greenwich Point. The entire absence, however, at present, of many 

 of the marine forms obtained in dredgings in the Delaware opposite the city, as at Smith's 

 Island, now removed, and in certain well borings at Pavonia, Pensauken, Gloucester and 

 other places in New Jersey, where the depth reached the old blue clay, indicates conditions 

 quite different from those now prevalent. In the Bay itself comparatively few living species 

 are found, at least in any abundance. 



In the study of local forms which follows, the district included may be considered as 

 circumscribed by the circumference of a circle having a radius of one hundred miles from 

 Philadelphia, containing the States of New Jersey and Delaware, the southeastern part of 



5 



