WISSAHICKON FORMATION 337 



are based on reconnaissance work on the part of Doctor Williams and 

 the writer. Throughout its entire extent no limestone has been found 

 up to the present time. It is, however, quite possible that small masses 

 of this material have been overlooked, as the area traversed is a portion 

 of the less dissected part of the Piedmont plateau, where the easy dis- 

 integration of the less resistant phyllites offers few exposures. The areal 

 mapping, however, is easily carried on from a study of the fragments, 

 which occur plentifully in the soil. 



The rocks constituting the phyllite portion of the Wissahickon forma- 

 tion are essentially sericitic, chloritic, and occasionally talcose schists, 

 which clearly show their sedimentary origin, and have been less meta- 

 morphosed than the Wissahickon schists already described. Two views 

 are held regarding their relations to the contiguous formation. They 

 may be regarded as an infolded younger series, as held, by the late Pro- 

 fessor Williams, or they may represent a less metamorphosed portion 

 of the Wissahickon formation. It seems probable that there is truth in 

 both views, and during recent years the impression has developed that 

 they represent the upper portion of the Wissahickon formation, which 

 has been less metamorphosed, but that they are not separated by any 

 great interval from the more crystalline Wissahickon schists which 

 border them on either side, and from which they cannot be separated 

 by any sharp line. When crossing the boundary between the two for- 

 mations one may recognize within comparatively short distances that a 

 boundary has been passed, but up to the present no contacts between 

 the two portions of the Wissahickon formation have been found. The 

 interpretation of the phyllites here discussed does not necessarily or 

 even probably apply to the more extensively developed phyllites lying 

 a little farther west, though some of the latter may be the equivalent of 

 the more eastern phyllite. 



The western phyllitic rocks composing the area between Parrs ridge 

 on the east and the sedimentaries lying at the base of Catoctin moun- 

 tain occupy a broad belt which enters the state of Maryland from Penn- 

 sylvania and crossess the state into Virginia. Detailed work on this 

 territory has not yet been completed, but the facts at hand would seem 

 to indicate that they rest on the Shenandoah limestone of the Frederick 

 valley, and are in turn overlain by the Triassic sandstones and shales. 

 Passing westward from the more crystalline rocks east of Westminster, 

 there appears to be a constantly decreasing amount of recrystallization 

 and metamorphism until along their western margin they present char- 

 acters similar to those of ordinary shales. Within this broad belt of 

 phyllites occur numerous narrow valleys of limestone or crystalline 

 marble and lenticular areas of chloritic and sericitic schists, which are 



XLV-lk'u.. (itoi.. Sue. Am., Vol. It;, 1904 



