1884.] NATURAL SCIENCES OP PHILADELPHIA, 239 



The Laurentian. — To this belongs the belt of hornblendic 

 rocks above the line of the Pennsjdvania R. R., which to the 

 west is narrow but which, rapidly bi-oadens to the east, as it 

 extends into Delaware county, Pa., and contracting again to a 

 narrow neck at Chester Creek, there connects with another 

 irregular area occupying all the northwestern portion of Delaware 

 county. This area connects with still another to the north, and 

 to the east of West Chester. This rock is a dark hornblendic 

 gneiss or amphibolite schist, dipping usually to the northwest, 

 rarely in the opposite direction. With it is associated a grayish 

 to bluish gray rock, usually finely crystalline, which has been 

 designated as diorite and syenitic granite by the Pennsylvania 

 geologists. Owing to the absence of petrographical facts con- 

 cerning this rock, however, nothing definite can be said concern- 

 ing it. It shades by indistinct degrees into the amphibolite 

 schists, the two varieties probably forming the same eruptive 

 series. 



The Gambro-Silurian. — This formation, so largely developed 

 through the counties of southeastern Pennsylvania, has one area 

 in the northwestern part of Delaware, and two smaller expo- 

 sures. In the northwestern area, a coarse quartzitic rock is 

 found to underlie a highly crystalline magnesian marble. These, 

 as we shall more clearly perceive further on, must be referred 

 respectively to the Potsdam and Calciferous, the latter of which 

 is equivalent to the Lower Magnesian limestone of the West. 



The Mica Schists and Gneisses. — To the north of the belt of 

 Laurentian gneisses, and resting upon the latter, is a series of 

 mica schists and granitic gneisses, with which are associated 

 bedded granites, serpentine, and hornblende rocks. They have 

 commonly been referred to the Mont-Alban, which, together 

 with the older hornblendic rocks, were called Azoic, the two 

 forming a part of the southern gneiss area as known by the 

 Pennsylvania geologists. It will be my aim, shortly, to show that 

 the hornblendic rocks and mica schists do not make two succes- 

 sive formations, within the Azoic, but that, while the former is 

 either Laurentian or Huronian, the latter must be placed above 

 the Trenton, and possibly above the Hudson River, slates. Their 

 exact position in the Palaeozoic scale, however, will probably 

 never be determined, owing to the complete absence of fossil- 

 iferous remains, due to the extreme metamorphism. The rocks 

 have been subjected to great contortion, the strata having been 



