1884.] NATURAL SCIENCES OF PHILADELPHIA. 255 



marl, we may call 60 feet a safe estimate for the thickness of the 

 lower marl bed. Of this total, more than one-half is represented 

 by the argillo-micaceous layer, and the remainder by the thin 

 shell layer, and the lowest cretoidal marl. There is one reason 

 why the thickness of the lower marl stratum, as developed at 

 Summit Bridge, should be employed in a calculation of the 

 thickness, rather than the smaller figures obtained farther to the 

 east or west. Summit Bridge is on the dividing ridge. From 

 this meridional line, the land slopes to the east and west. The 

 marl, therefore, offers a diminishing thickness of outcrop as the 

 river is approached, owing to the erosion of the upper argillo- 

 micaceous stratum, which, in the neighborhood of St. George's, 

 and thence to Delaware City, has been thinned down to a thick- 

 ness of only a few feet. It is for this reason that the upper 

 portion of the lower marl bed is so extensively developed at the 

 western end of the belt, while the lower portion of the same for- 

 mation is confined to the eastern portion of the belt. Since the 

 upper argillo-micaceous stratum is a poor, and even objectionable, 

 material for fertilizing purposes, while the contrary may be said 

 of the lower cretoidal variety, the locality for marl diggings 

 must lie east of the railroad bridge. 



Induratep Marl Bed. 



The northern limit of this belt, which is also the southern 

 limit of the lower marl bed, starts near the mouth of Scott's run, 

 and thence keeps parallel with the canal to the railroad, when it 

 begins slightly to diverge, cutting the headwaters of the northern 

 branch of the Bohemia River. The southern limit of the belt 

 can only be approximately outlined, but as can best be determined 

 runs from Port Penn through the headwaters of Drawyer's 

 Ureek, and crosses the Maryland line four miles below the head 

 of Bohemia River. The indurated marl stratum is divided into 

 two layers, the lower red sand, and the upper indurated marl. 



Bed Sand. — The formation which has been called the indurated 

 marl bed is the equivalent of the red sands of the New Jersey 

 geologists, it being, in both cases, the prominent parting layer 

 between the lower and middle marl. Along the south side of the 

 canal, between the railroad bridge and St. George's, a soft reddish 

 yellow sand of uniform character rests upon a stratum of black 

 marl, It is deyeloped to a considerable thickness in the neigh- 



