18S2.] 511 [Frazcr. 



The writer takes issue with Islr. Hall, as will appear further on in his 

 statement, as to the absence of large masses of schist, in contact with the 

 Potsdam and with the Laurentian north of the Chester valley.* 



Mr. Hall's argument is virtuallj^ as follows : 



(1.) " The Philadelphia, Manayunk and Chestnut Hill beds or the South 

 Valley Hill, wJuch is equivalent to part of them, cannot be loicer than the 

 Laurentian {Third Belt of Rogers)." 



Tills will be universalh' conceded. 



(2.) "It is clear that the Potsdam sandstone was deposited on this Third 

 Belt." 



This is not clear except, perhaps over a limited area. It is not true of 

 the Potsdam in Lancaster, nor is it true of the Potsdam in Southern Ches- 

 ter, nor in parts of Northern Chester. For instance, the evidence that 

 the Potsdam, between Doe Run and Toughkenamon, underlies the lime- 

 stone and overlies the chlorite schists of that region is very strong. If 

 the limestone interposed between the quartzite and the schists, then a 

 border of limestone should show on the east and west ends along the 

 iriegular boundary of the Potsdam area, but it does not. 



A series of small detached exposures of limestone stretch east by north 

 from the Doe Run limestone and likethelattershownotraceof Potsdam on 

 their northern edges. These as well as the Doe Run limestone, are held to 

 be older than the Potsdam, because the dip is S. or S. E. continuously 

 from the South Valley Hill southwards, decreasing in intensity in that 

 direction, so that if not monoclinal the structure must be considered 

 anticlinal, and cannot be syncUaaJ. The meaning of this is that the Doe 

 Run limestone is younger than the crest of the Valley Hill, and that its 

 southern edge is younger than its northern edge (since the preponderance 



any means as clear as in the region north-west. It would he perfectly easy, as 

 there pointed out, to place the Peach Bottom slates above the quartzite without 

 deranging the structure of the upper region, as therein suggested. Jbe objec- 

 tion to placing the series above the limestone, i.e., tliat no limestone appeared 

 hetween the gentle axis of Tocquaii creek and the slates, of course would not 

 be an objection to tliose wlio credit the Tocquan schists tliemselves with being 

 above tlie limestone. 



Two explanations of Hudson River slates at Peach Uottom are possible with- 

 out changing the horizons of the measures to the X. \V. One is the omission 

 liere altogether of the limestone in the series. The otlier (held by Prof. Burrois, 

 who visite<l the region), a fault line north ot the slate belt. 



It is only fair to admit, however, that the Hu ison River age of these quarries 

 is not proven. 



*The discovery of Mr. Lewis as to the two kinils of scratches nuide by the ice 

 and the creep, must be regarded as an important application of the reasoning of 

 the Scotch geologists to our own country. In some cases Mr. Peach and Mr. 

 llorne have been able to distinctly ascribe thi-ce distinct lines of markings to 

 movements of very different age. 



The colors on the geological maps are somewhat confusing. The dark red. 

 which in the scale is called the intermediate Manayunk belt, seems to be ap- 

 plied on the map to the northerlj- Chestnut Hill group, and vice versa. 



