254 PROCEEDINGS OF THE ACADEMY OP [1884. 



same, rising about a foot above the surface of the water, and, 

 farther west, to a height of 20 feet. The several subdivisions of 

 the lower marl will be treated in the order of their age. 



Gretoidal Marl. — This lowest layer is a tough, bluish black 

 marl, which, upon drying, turns to a lighter ashen or earthen 

 color, when it is found to be made of a large amount of green 

 sand, siliceous sand and argillaceous matter. The solid particles 

 are coated with chalky carbonate of lime, which, under the 

 microscope, appears as a fine white powder of a granular char- 

 acter, but often light and flocculent. 



Shell Layer. — The cretoidal marl is always found beneath a 

 layer of shells or shell marl, having a thickness of about 3 feet. 

 In fact, the shells are usually mixed with a greater or less 

 quantity of the black earthy marl having the characters of the 

 cretoidal variety. The shells are usually the characteristic 

 species of the New Jersey equivalents, the most abundant being 

 the clumsy Exogyra costata with Pychnodonta vesicularis and 

 Ostrea larva. 



Black Argillaceous and Micaceous Marl. — This layer, over- 

 lying the shells, in its lower part possesses somewhat the charac- 

 ters of the cretoidal marl. To the west of the Delaware railroad, 

 however, it rises well out of the water of the canal, and assumes 

 a distinctly argillaceous nature, becoming a black micaceous clay. 

 It also shows an entire absence of calcareous matter, and pos- 

 sesses a decidedly styptic taste, due to the large quantity of 

 sulphate of iron in the percolating waters. This argillaceous 

 marl, when examined in the dry state by a glass, is found to be 

 composed of minute sharp glassy particles of quartz, coated with 

 a grayish dust, and associated with a few green sand particles of 

 unusual fineness, together with a considerable quantity of minute 

 scales of muscovite. 



Thickness. — Fortunately we have sufficient evidence for meas- 

 uring accurp,tely the thickness of the lower marl bed. The belt 

 has an average breadth of 1^ miles, which, with a dip of 40 feet 

 to the mile, would give a thickness of 60 feet for the stratum. 

 At Summit Bridge, the black marl outcrops to a height of 42 to 

 47 feet. ^Calling it 40, and adding this to 15 feet of marl in 

 Higgin's pit, on much lower ground between St. George's and 

 Delaware City, we get a total of 55 feet. Since the 15 feet 

 excavation at the latter place f?iiled to entirely peijetrate the 



