178 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Paoli and on Henderson and Bridgeport hills. The Henderson and Bridgeport outliers, while 

 Uthologically similar to the main mass of mica schist, can not be positively correlated with the 

 formation. Their relation to limestone of Chazy age, as seen in the Schuylkill River cut, is such 

 as to admit interpretation either as an interbedded structure or as the overlying synclinal 

 structure which the Octoraro schist must possess. 



The mica schist is characterized by a pronounced lamination of a slaty rather than a schist- 

 ose type. The laminae exhibit lustrous silvery surfaces and a blue-gray or green-gray color, 

 which under the action of weathering alters to reddish yellow. 



The chief constituents of this mica schist are quartz, muscovite, orthoclase, and chlorite. 

 Quartz occurs in interlocking grains which show undulatory extinction and other pressure effects 

 in their form and arrangement. Orthoclase occurs sporadically in considerable areas, but it is 

 not an important or characteristic constituent. Chlorite is uniformly distributed through the 

 rock, interspersed with wavy lamellae of muscovite. Plagioclase, biotite, magnetite, ilmenite, 

 tourmaline, apatite, and pyrite are accessory constituents. 



The constituents of the schist do not possess clearly defined outlines and the crystalline 

 texture is neither so coarse nor so sharply defined as in the Wissahickon gneiss. 



In the hand specimen quartz is completely overlain by minute plates of mica, which alone 

 show on the cleavage surface, while eyelets of quartz may show on the edges of the laminae. 

 Cubes of pyrite more or less altered to limonite are characteristic. Considerable oxide of iron 

 is present in this formation, which is evidently the source of the hmonite ore that occurs sporad- 

 ically in pockets between the limestone and the schist. 



The analyses of the mica schist, with the relatively high alumina and low silica and absence 

 of lime, fairly indicate a sedimentary origin for the formation. 



The lower beds of this formation are more calcareous, more siliceous, finer, and darker 

 colored than the upper beds. The outlying areas at Bridgeport and Henderson are of this 

 character. In some places there appears interbedded with the mica schist a quartz schist or a 

 quartzite similar to the Cambrian quartzite. Fragments of such a quartz schist show 1 mile 

 north of Paoli on the northwestern boundary of the mica schist, and 2 miles north of Wayne 

 such a siliceous member furnishes sand for local use. 



The mica schist of the South Valley Hills appears to overlie the Shenandoah limestone 

 without faulting or unconformity. This is indicated by the lithologic gradation between the 

 limestone and schist which may be seen at the northwest base of the South Valley Hills and by 

 the persistence of the same geodiferous quartzose beds along the contact of the two formations 

 and is confirmed by the fact that the outcrops show that deformation was by folding and flowage 

 and not by faulting. 



The structure of the hills is evidently synclinal, though cleavage and fissility are so pro- 

 nounced as to obscure the stratification. On the limbs of the syncline cleavage and bedding are 

 approximately parallel, while cross structures prevail in the trough of the syncline. 



The structure of the formation in the South Valley Hills indicates a thickness probably not 

 exceeding 1,000 feet. It is not known, however, whether the full thickness of the formation is 

 present here. 



*'* * * * * * * * 



The formation is held to be Ordovician in age on the ground of its normal stratigraphic 

 position on the Cambro-Ordovician limestone. It is correlated with the Berkshire schist of the 

 New England section, the Hudson schist of New York, and doubtfully with the phyllites of 

 Maryland. It is the Primal upper slate of the First Geological Survey of Pennsylvania, and the 

 Cambrian phyllite of the Second Geological Survey. 



Ranging southwestward from Orange County, N. Y., the Cambrian and Ordo- 

 vician strata pass into New Jersey with the facies described as the Hardyston 



