1882.J 51'^ IFrazer. 



caceous minerals composing tliem, Mr. Hall does not stale the nature of 

 the fragments forming the conglomerate, bj.it on page 46 the significant 

 statement is made that, " Itacolumite and hydro-mica schist have been ap- 

 plied to the specimens annlj-zed." 



There can be no error as to the rocks thus spoken of, as appears from 

 six field numbers which are given of specimens of Potsdam analyzed, of 

 ■which the first two are found on referring to the analyses to be "Itacolu- 

 mite" and the last four "Hydro-mica schist." It will hence be unneces- 

 sary to multiply examples of the same kind which might be taken from 

 any of the four counties enumerated above. The fact is indisputably es- 

 tablished by Mr. Hall himself that remains of the schists are abundantly 

 found in the Potsdam. 



At this point the simple statement is made that the same difficulties are 

 encountered in trying to find a place for the schists until the upper limit 

 of the limestone is passed. As it. is well known that there is an abun- 

 dance of slates above this limit, the inference is drawn that the schists 

 belong there. 



This part of the discussion may be left Avitli the remark that to the 

 knowledge of the writer no extensive series of cJiloritic schists has been 

 found to belong to the measures which are without dispute above the 

 limestone of II. 



A brief resume of the principal reasons for assigning to tliese schists a 

 lower horizon may be here roughly sketched : 



(1.) There can be no doubt that the straight and narrow valley called the 

 Chester Valley is connected actually with the great Lancaster limestone, 

 and that it represents a part of a synclinal fold. The anticlinal once con- 

 necting it with the larger mass of limestone passed ovet- (and proba1)ly high 

 over) all of northern Chester county. If the schists to the south of the 

 valley lie on the limestone, then the entire thickness of the latter must 

 plunge beneath the surface within the limits of the valley. At places (as 

 between Atglen and Pomeroy), the actual space which may be filled by 

 limestone varies from a few hundred to fifteen hundred feet. But the 

 limestone as measured on the Nefisville and Wrighlsville sections is about 

 2700 feet thick. Of course if there be an upthrow on the south, any 

 amount of the upper part of the limestone may have been eroded and any 

 small portion of the lower beds left. 



The dips are northward along the western part of Sadsbury township ; 

 and they are in sandy mica schist and gneiss on the north side [as for ex- 

 ample N. 103 W.-303 (Atglen) ; X. 450 W.-IO^ ; N.-SO^ (near Parkes- 

 burg) ; N. 4.)0 "^.-40^ (ditto)]. The limestone when first found in place 

 by the machine shops in Parkesburg strikes E. 25^ N.- vertical. Further 

 east near Pomeroy it is on the northern edge of the valley N. 10° ^Y.-50'^. 

 Decomposed gneiss just north of Pomeroy gives a succession of S. E. dips 

 about S. 10° E.-So^. A few hundred feet south of the north dip in the 

 limestone is a dip zb S.-80^, and a thousand feet or so in the same direc- 

 tion S. 15° E.-60- etc. 



PROC. AMER. PHILOS. SOC. XX. 112. 3>r. PRINTED JANUARY 22, 1883. 



