ORDOVICIAN ROCKS 301 



It is part of the great Shenandoah belt of limestone, the "Auroral 

 limestone " of H. D. Rogers, and " formation number II " of the Second 

 Geological Survey of Pennsylvania. 



Aside from a few scattered and minor exposures, the limestone of the 

 southeastern belt is confined to and controls the form of Chester valley, 

 a conspicuous topographic feature of this district. For this reason it 

 has long been locally known as the Chester Valley limestone. This 

 name is retained for the formation. 



ORDOVICIAN ROCKS: WISSAHICKON MICA-GNEISS (MICA-SCHIST AND MICA- 

 GNEISS) 



Areal distribution. — The formation overlying the Cambro-Ordovician 

 limestone occurs in two areas, separated by the Buck Ridge complex of 

 pre-Cambrian gneiss and gabbro. One area lies to the northwest and 

 the other to the southeast of Buck ridge. 



The lithologic character of the formation differs somewhat in the two 

 areas. In the northwest it is characteristically a mica-schist and in the 

 southeast a mica-gneiss. 



Distribution of mica-schist. — The mica-schist is chiefly confined to the 

 south Chester Valley hills, expanding to the southwest, where the hills 

 cease to be a distinct topographic feature and where Buck ridge pitches 

 under the mica-schist and mica-gneiss, which here graduate into each 

 other. Outliers of the mica-schist occur north of Berwyn and of Paoli 

 and on Henderson and Bridgeport hills. The Henderson and Bridge- 

 port outliers, while lithologically similar to the main mass of mica-schist, 

 can not be positively correlated with that formation. Their relation to 

 limestone of Chazy age, as seen in the Schuylkill River cut, is such as 

 to admit of the interpretation either of an interbedded structure or of an 

 overlying synclinal structure, such as the mica-schist must possess. 



Character of the formation and stratigraphic relations. — The formation of 

 the northwestern area is lithologically a quartz-muscovite-schist, which 

 frequently contains considerable iron hydroxide. When fresh it exhibits 

 a lustrous silvery surface and a blue-gray or green-gray color. On weath- 

 ering, the color alters to a reddish-yellow and the rock splits readily into 

 folia?. The quartz is completely wrapped about by the mica, which alone 

 shows on cleavage planes. This formation is evidently the source of the 

 small pockets of limonite ore which are found between it and the lime- 

 stone. 



The chief constituents of this mica-schist are quartz, muscovite, ortho- 

 clase, and chlorite. Quartz occurs in interlocking areas, which show 

 undulatory extinction and other pressure effects in their form and ar- 

 rangement. Orthoclase occurs in considerable areas, but it is not an 



