Frazer.] 4UU [Dec. 4) 



rectness of ascribing to this formation the iron ores which are found in the 

 schists immediately above the quartzite.* 



The Grubb ore bank (No. Ill of the map) is the only one which lies 

 wholly within the area of the Hellam quartzite as given on the map, but 

 a reference to the description of this bank (Vol. C, p. 64, 2d G. S. of P.) 

 leads to the belief that the larger part of the ore lies in a small remnant 

 of the bottom schists of the next higher formation, which has escaped the 

 erosion that cut off the higher layers of that formation. Part of it, how- 

 ever, answers to the description of an iron ore which may really 

 belong to the quartzite and which has been noticed in the rocks forming 

 the outer casing of the South mountain .f 



Siluric. (s) 



The York Limestone and Schists {Auroral of Rogers, in part the Cal- 

 ciferous Sand rock of the New York Survey). This important member of 

 the Palaeozoic series in York county consists of at least two, and perhaps 

 three, distinct kinds of rocks, and inasmuch as the kind that occurs at the 

 bottom (which resembles strongly that which occurs among the limestone 

 beds themselves, and also above them) has already been mentioned several 

 times by anticipation, it will be advisable to consider it first. 



Hydro-mica Schists, (sj) 



It was previously stated that Rogers, and following him, almost all other 

 writers on geology up to the commencementofthe Second Geological Survey 

 of Pennsylvania, had given the name of " talcose slates " to a group of rocks 

 which he connected in epoch with the quartzite. The word talcose was ap- 

 plied to them because from their softness and greasy feel it was assumed that 

 they were largely composed of "talc;" but subsequent investigations of 

 these rocks in the chemical laboratory have shown tnat they contain little 

 or no magnesia, and that they derive their peculiar characters from large 

 amounts of a group of micas containing potash or soda and water. Prof. 

 James D. Dana conceived the happy thought of naming the group the 

 " Hydro micas " (or water-containing micas), and naturally the rock 

 which is mainly composed of them is called Hydro-mica schist. 



These hydro-mica, or nacreous schists, are not of uniform appearance. 

 Sometimes, and especially in the beds that underlie the limestone, they 

 are firmly compacted together, making hard rock masses and high hills, 

 as at many places along the Susquehanna, from Wrightsville to Cabin 

 Branch run, and elsewhere in the county. Sometimes they are so much 

 disintegrated as to form dust, which on close view is seen to be mainly 



*Of course, if the Potsdam have an upper member consisting of schists, the 

 above assignment is correct ; but I know of no instance in which the opposite 

 supposition is not equally supported hy the (acts. It is also to be noted that 

 the limestoneand iron-ore bearing schists are more frequently found together 

 without the quartzite, than the quartzite and schists without the limestone. 



tCottrel I, Benson's and Smyser's mines (Nos. II and 112) are ou the borderline 

 between the quartzite and limestone. 



