ORDOVICIAN ROCKS 303 



of the formation by earlier surveys into three formations — the Chestnut 

 hill, Manayunk, and Philadelphia gneisses. 



Character of the formation and stratigraphic relations. — With local varia- 

 tions, which are later indicated, the formation may be described as a 

 medium to coarse-grained gneiss, characterized by an excess of mica. 



The chief constituents of the formation are quartz, feldspar, both ortho- 

 clase and plagioclase: green or brown biotite, and muscovite. Magnetite, 

 apatite, zircon, tourmaline, garnets, andalusite, sillimanite, and zoisite 

 are accessory constituents. The more gneissic beds contain abundant 

 orthoclase and plagioclase, which, whenever tested, proved to be an acid 

 variety between oligoclase and andesine (A^An^. The rock is perfectly 

 crystalline, the quartz fresh, and occurring in interlocking areas. The 

 freshness of the crystallization and the absence of pressure effects on 

 the constituents indicate a recrystallized sediment. 



The belt passing through Chestnut hill and Bryn Mawr is composed 

 of beds which are alternately micaceous and quartzose, and often bear 

 a marked similarity to the mica-schist of the south Chester Valley hills 

 (see plate 53). It is also very garnetiferous in the neighborhood of the 

 peridotite intrusives, and for this reason has been called the ''garnetif- 

 erous mica-schist. 1 ' Even in this belt the formation is not free from 

 feldspar, and in the direction of Manayunk and Philadelphia massive 

 gneissic strata interbedded with micaceous and quartzose layers become 

 increasingly prominent, and garnets are replaced by andalusite and 

 sillimanite. 



Within the contact zone of the most northerly of the serpentine dikes 

 the mica-gneiss has locally altered to a muscovite-schist. It contains 

 large lustrous areas of muscovite, which have won for it the designation 

 " spangled mica-schist." It is silicious, light colored, coarse^ crystal- 

 line, and splits readily. 



The rock possesses a chemical composition, as shown by analysis,* 

 which resembles that of a slate, and bears out the field and petrographic 

 determination of the formation as of sedimentary origin. 



The high alumina percentage, low alkali percentage, and the pre- 

 ponderance of magnesia over lime shown by these analyses are all char- 

 acteristic of silicious argillites, while in all feldspar-bearing igneous 

 rocks, on the other hand, lime dominates the magnesia. 



* Bascom : Op. cit., p. 116. 



