112 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



tively be stated to be of Georgian age. It can, however, be safely affirmed to be Cambrian 

 and is to be correlated with the Cheshire quartzite of New England, the Poughquag quartzite 

 of New York, the Hardyston quartzite of New Jersey and provisionally with the Setters quartz- 

 ite of Maryland. It is the Primal sandstone of H. D. Rogers and the Formation No. 1, Chickies 

 sandstone of the Second Geological Survey of Pennsylvania. * * * 



The Shenandoah ("Chester Valley") limestone is a heavily bedded crystalline white or 

 blue magnesian limestone. Its surface exposure in the Philadelphia district is confined, with 

 a few scattered outcrops along the Huntingdon and Cream Valley faults, to Chester Valley, 

 where it covers an area 20 miles long and 2 to 2^ miles wide. * * * -phe limestone is highly 

 siliceous and magnesian. The analyses show great variation in the percentages of SiOj and 

 MgO, but no analysis gives a sufficiently high content of MgO to warrant calling the formation 

 a dolomite. It is everywhere crystalline, and increasingly so from west to east. Associated 

 with increasing crystallinity is a lighter color, though blue and white limestone may occur in 

 the same quarry. 



It is in places quite micaceous, and always so in the neighborhood of the overlying mica 

 schist. The beds immediately underlying the mica schist are siliceous, micaceous, and schistose 

 and are to be characterized as calcareous schist. The limestone is abundantly traversed by 

 calcite and quartz veins. Quartz, feldspar, phlogopite, graphite, pyrite, and siderite are acces- 

 sory constituents, disseminated in minute grains and crystals. Limonitic iron ore occurs in 

 pockets in the limestone. 



Intercalated With the limestone are beds of siliceous or micaceous schists. These interca- 

 lations, which are lenticular in character, are conspicuous in the hmestone west of the Phila- 

 delphia district but occur infrequently in that portion of the limestone confined to Chester 

 Valley. Near Pomeroy, 25 miles west of the Philadelphia district, an intraformational cal- 

 careous conglomerate shows near the top of the /formation. * * * 



If the interpretation of the structure given above is correct the thickness of the formation 

 must be much less than the width of its outcrop. It is not determinable exactly, but probably 

 is not greater than 1,000 feet. 



Fossils of Chazy, Beekmantown, and Trenton ages have been found in the limestone 

 occurring to the west of Chester Valley and stratigraphically continuous with the limestone of 

 Chester Valley. Fossils have also been found in Chester Valley in somewhat ambiguous material. 

 This material is a drusy, geodiferous rock which seems to have originated through the replace- 

 ment of calcareous material by silica. A mass of the rock is exposed just south of Bridgeport, 

 near the Trenton branch of the Philadelphia Railroad. Elsewhere it is found only in scattered 

 fragments which rest on the limestone and accompany more or less persistently the contact of 

 limestone and Octoraro schist. It thus seems to mark a definite horizon whose persistence 

 between the schist and the limestone precludes the possibility of a faulted or an uncoirformable 

 contact. 



The material has not proved fossUiferous except at one locality, near Henderson station, 

 where fragments resting on the surface of the ground have been found to contain gastropod 

 and cephalopod forms. 



The following determinations were made by E. 0. Ulrich, of the United States Geological 

 Survey: Raphistoma, two species, Maclurea, Lituites, Cyrtoceras. These are Ordovician forms 

 and indicate a horizon in the lower half, probably Beekmantown. The limestone overlies con- 

 formably Georgian ("Lower Cambrian") quartzite and is therefore Cambro-Ordovician in age. 

 It is correlated with the Stockbridge limestone of New England and New York, doubtfijiy 

 with the CockeysviUe marble of Maryland, and with the Shenandoah limestone of Virginia. 

 It is the most easterly representative of the great belt of limestone, the Auroral limestone of, 

 H. D. Rogers and Formation No. II of the Second Geological Survey of Pennsylvania. Aside 

 from a few scattered and very minor exposures, the limestone of the Philadelphia district is 

 confined to and controls the form of Chester Valley, a conspicuous topographic feature of the 

 district. For this reason it haS long been locally known as the "Chester Valley limestone.'' 



