1884.] NATURAL SCIENCES OF PHILADELPHIA. 241 



Thence it runs close to the south shore of White Clay Creek to 

 a distance of two miles beyond Roseville, where it makes an 

 abrupt bend to the north, until at Stanton the rocks cease 

 to be found. A mile back of the railroad station, they again 

 appear, continuing to a point about a mile back of Newport, 

 where their course turns slightly to the southeast, crossing 

 the Wilmington turnpike just before it is intersected by the 

 Wilmington Northern R. R., thence it follows the turnpike 

 through the southern half of the city, keeping just north of the 

 Pennsylvania R. R., to a point south of Bellevue, where the 

 line cuts the river. From there the Delaware River marks the 

 southenj boundary. The area as above indicated may be divided 

 into two pretty distinct belts : (1 ) the southern belt of horn- 

 blendic rocks, and (2) the northern belt of micaceous rocks, 

 with which are associated interstratified beds of coarse-grained 

 orthoclase granite, feldspar, quartz and quartzite. 



The boundary line between these two belts can be traced very 

 accurately, and is found to correspond pretty closely with the 

 lines of strike. Beginning with the western boundary of the 

 State, the line follows approximately the coui'se previously 

 traced out, but one-half a mile to the north of the same. It 

 continues thus to a point south of Milltown, when an abrupt 

 turn to the northeast is taken, the line crossing the Brandj^wine 

 only a few miles from the head of the State. It is owing to the 

 northerly course of this line, as compared with the southern 

 limit of the Azoic area, that the northeastern portion of the 

 State is covered more largely with the hornblende, and the' 

 northwestern with the micaceous rocks. 



The Lithology of the Crystalline Rocks. 

 The rocks which cover the crystalline area may be classed as 



follows : — 



( Granite, 



Micaceous < Granitic gneiss, 



l Mica schist. 



Amphibolite schist. 



Hornblendic \ T^^ ^ 



Blue to gray trap. 



/ Marble, 

 Calcareous < r, , • i i t 



[ baccharoidal limestone. 



Serpentine. 

 Quartzite. 

 Vitreous quartz. 



