Frazer.] Oi-j (Deo. lo, 



of southerly dips continues across the belt). On its southern edge rests 

 the Potsdam in W. Marlboro' township, still with a south dip {i.e., S. 10^ 

 E.-45-; S. 50 E.-70O; S. 20^ W.-40O ; E.20O S.-40O, izc, &c.), that rap- 

 idly becomes gently undulating and almost horizontal : and this structure 

 continues to tlie Dehiware line. 



The axes of the Chikis anticlinal folds can be seen to be mica schist of 

 similar ciiaracter to that of the South Valley Hill. 



The rock underlying tlie possible Potsdam quartzites in the lower Sus- 

 quehanna, are clearly of the same character and series. 



The Potsdam in York county is seen to overlie the same schists near 

 Wrightsville and York, near the former of which, as if to settle all doubt, 

 two or three folds bring to the surface within a short distance all the meas- 

 ures above and below it. The Potsdam of Franklin county which lies upon 

 the South mountain covers these same schists, and the very numerous 

 varieties of clays and associated iron ores which are due to their decompo- 

 sition. 



The Xorth Valley Hill quartzite in Sadsburj', Valley, East and West 

 Brandywine, Upper and Lower Uwchl:.n, and other townships, is pre- 

 ceded and succeeded by gneissoid and chloritic mica schists, as seen 

 at Atglen, Pomeroj^ * Stottsville, Sadsburyville, north of Downing- 

 town, on the Brandywine, north and south of Lionville, and at otlier 

 places. 



In this connection, the following, taken from the notes which were made 

 b}' Mr. Hall and the writer, when, in September, 1876, they visited to- 

 gether Harper's Ferry, and made a section of the Potomac river in its 

 vicinity, may not be without interest. It is necessary to premise that ]\Ir. 

 Hall holds the opinion, which is tlie natural deduction from his views of 

 the horizon of the South Valley Hill schists, that the rock which the 

 writer has designated "Mountain Creek Rock" from its occurrence in the 

 part of the South mountain which is contiguous to this stream, is a repre- 

 sentative of the Potsdam. 



The exposure at the head of the bridge on the ^laryland side, opposite 

 Harper's Ferry, is of a great niass of this schistose rock with fragments of 

 pink quartz, dipping S. 30° E.-450. This continues for an horizontal dis- 

 tance of 14(51 feet (44.') meters) east and west of the bridge, along the 

 Potomac river. 



To the west there appears an hydro-mica schist, dipping S. 40^^ E.-18-, 

 but curling so as to render it difficult to ascertain the true dip. 



Further west are met in succession : 

 f Greenish chlorite slates. 

 j Hydro-mica slates very much convoluted. 



Hydro-mica slates. 

 [ Chlorite slates dipping E. 20^ S.-SSC'. 



All the above have practically one dip. 



* Stottsville, wliioh is omitted from the geological uiup ol Chester county, is 

 on the southern side of the valley ojiposite Poineroy. 



A 



