A STUDY OP ONE POINT IN THE ArCIDEAN-PaLjEOZOIC CONTACT LINE IN SOUTH-EASTERN PENNSYLVANIA. By PeRSIFOR FrAZER. Docteur es-Scienees Naturelles (University de France), Phila delphia, Pa. [ABSTRACT.] The crystalline rocks in and near Philadelphia were classified by Rogers into three belts of which he thought that two, the northerly and the southerly, might be the same ; and the middle, above them. He never doubted that the Chester Valley limestone was far above all these gneisses and mica schists, and the hydro mica-schists which formed his Upper Primal series. Since then disputes having arisen in various parts of the world as to the stratigrapliical relationships of certain belts of crystalline rocks near the contact line of palaeozoic and archaean, Philadelphia was not wanting in defenders of the theory that the schists and gneisses, forming its foundations, were laid down on the lime stone known formerly as No. 2, or the Auroral, which is universally ascribed to the Lower Silurian Age. These suppositions have often been vaguely heard as corollaries to the statements of orders of superposition in other parts of the United States, and of the world ; but have been presented tan gibly, so far as the writer knows, by Mr. Charles E. Hall, in one pub lication only, viz., “C„” of the volumes of the 2nd Geological Survey of Pennsylvania. Without at this time sketching the full data or bearings of Mr. Hall’s views it may be said that he deemed the proof of his theory of structure to be more fully illustrated at that portion of the course of the Schuylkill near Conshohocken than at any other point. This, as he remarked, was the key to the whole structure of the southern Chester Valley ; and this implied that if the superposition of the limestone could be clearly shown here, there was no other place where it would present equally serious difficulties. The reason of this is plain. This region exhibits a sharp folded outcrop ; so sharp indeed that it would seem that but one hypothesis of structure could successfully account for the four alternations of schist and limestone on a short section line. Mr. Charles E. Hall’s views of this structure as published in the just cited volume C 6 of the Second Geological Survey of Pennsylvania, are found on p. 32 ; where also a small section from Henderson’s Station to Mechanicsville gives graphic form to his hypothesis. (3)
