1884.] NATURAL SCIENCES OP PHILADELPHIA. 251 



constructed with a view of showing the relative thickness of each 

 of the groups. In the classification the plastic clays have been 

 placed in the Lower Cretaceous, and are probably the exact 

 equivalent of the Wealden, while the marl deposits, ranging from 

 the Lower to the Middle Beds, can, upon palseontological 

 grounds, be referred with considerable confidence to the Upper 

 Cretaceous, or Chalk. The Sand Marl formation can at best be 

 placed in but an intermediate position, but is probably nearly 

 akin to the marls, all of the . fossils ever found within the sand 

 marls of New Jersey having been the characteristic species of the 

 marl beds proper. 



Structure. — The different subdivisions of the Cretaceous foi-m 

 uniform beds, dipping at a low angle to the southeast. Having 

 been successively deposited upon a gently sloping bed, they 

 have remained in the same position, with no subsequent disturb- 

 ance. The general direction of strike can be seen from the 

 trend of the various belts. A line joining the point of contact 

 between the Cretaceous, at Newark, with a similar point back of 

 Newport, upon nearly the same level, ran N. 72° E., which 

 course, being approximately parallel with the lines of the lower 

 belts, may fairly represent the strike of the formation. The dip 

 was determined with great accuracy at Summit Bridge, about 

 midway of the width of the Cretaceous. At this place the canal 

 excavation has reached a depth of 10 feet, and continues several 

 miles to the westward, with gradually lowering banks. Upon 

 both sides of this wide cutting, the marl outcrops as a well- 

 defined laji-er, and as any number of lines in the direction of the 

 dip can be obtained, the amount of pitch can be accurately deter- 

 mined. As the transit could always be placed upon either bank 

 in front of the escarpment of marl, and the other side could be 

 easily seen, the operation of determining the difference in level 

 between the two outcrops, on a line running S. 30° E., was not 

 difficult. This difference in level, combined with the angle of 

 depression, as determined by the vernier, would give the data for 

 ascertaining the distance between the outcrops, the latter of 

 which varied from 300 to 400 feet. The results of the observa- 

 tions give a dip of 45 feet to the mile. 



Plastic Clays. 



This formation is the thickest member of the Cretaceous. Its 

 southern line begins a few miles south of New Castle, and extends 



