1884.] NATURAL SCIENCES OP PHILADELPHIA. 249 



the Lower Magnesian limestone of the West. The magnesian 

 marble which outcrops beyond the State line, at Nivin's, runs into 

 Delaware, and appears at the surface in the Jackson quarry at 

 Hockessin. Here the rock forms a clearly defined anticlinal fold. 

 The bending, both to the northwest and southeast, being observed 

 within the cutting. The limestone is overlaid by the mica schists 

 which, to the north of the pit, dip to the northwest, and to the 

 south, southeast, forming an anticlinal fold capping the limestone. 



To the southeast of Centreville, the limestone occupies the 

 same stratigraphical position as in the case just mentioned. At 

 Pleasant Hill the bending of the schists over a saddle of limestone 

 is beautifully shown in the quarry cutting, furnishing clear proof 

 as to the superior position of the mica schists. 



The Mica Schist and Gneiss. — The inference that these rocks 

 were primal, was based largely upon their lithological similarity 

 to many of the older crystalline schists. They were hence referred 

 to the White Mountain, or the Rocky Mountain, series. But litho- 

 logical similarities must invariably bend to higher statigraphical 

 evidence. In Delaware the micaceous rocks overlie the limestone, 

 and no readjustment of position can make the arrangement other- 

 wise. They are, therefore, younger than the limestone, which, in 

 its turn, is younger than the underlying Potsdam. 



The Calciferous limestone can hardly be referred to any other 

 position, and, invariably underlying the schists, the latter must 

 begin somewhere in the Silurian, and, possibly, mount as high up 

 as the Devonian. 



The Cretaceous. 



Geographical Extent. — The Cretaceous of Delaware, a continu- 

 ation of the same formation as developed in New Jersey, extends 

 across the State as a northeast and southwest belt, with a 

 breadth of 18, and a length of from 15 to 20 miles, covering a 

 total area of about 250 square miles. The northern limit of the 

 belt, laid down on the map, has already been traced out as marking 

 the southern boundary of the Laurentian. The southern limit 

 runs a little to the south of, and parallel with, Appoquinimink 

 Creek, cutting through the centre of Noxontown mill-pond, and 

 thence proceeds in a straight southeasterly direction. 



Subdivisions. — The formation may be divided into a number ot 

 subdivisions, based upon lithological grounds, the period ot 

 sedimentation extending through the whole Cretaceous ; i. e., 

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