472 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



sandstone and Magnesian limestone, conformable with both, and that 

 they therefore cannot belong to the " Taconiau " of Hunt, as the latter 

 had stated. Professor Prime also says : — 



" It is well here to emphasize the fact that these brown hematite ores all 

 belong to the Lower Silurian limestone foi'mation, since, in 1875, Dr. Sterry 

 Hunt, after a cursory examination of Ziegler's Mine in Berks County .... 

 made the mistake in a paper on ' The Decay of Crystalline Rocks ' before the 

 National Academy of Science, of supposing that the hydromica slates l)elonged 

 to the Huronian Period: — a mistake into which so eminent an observer as 

 himself would never have fallen had he been better acquainted with the 

 region." (Proc. Am. Phil. Soc, 1878, XVII., pp. 248-254 ; Am. Jour. Sci., 

 1878, (3) XV., pp. 261-269.) 



Mr. Charles E. Hall thus indicates the formations of Eastern Penn- 

 sylvania, in their ascending order (Proc. Am. Phil. Soc, 1880, XVIII. , 

 pp. 435-443) : — 



First. A series of granitoid, syenitic, quartzose, and micaceous schistose 

 rocks. 



Second. A series of syenitic, hornblendic and quartzose rocks. He states, 

 however, that " this series may be the upper members of the first." 



Third. Potsdam sandstone, conglomerate, quartzite, and occasional schis- 

 tose beds. " This, the Potsdam sandstone, rests uuconformab]}' upon the pre- 

 ceding two groups. The unconformity is seen at points east of Willow Grove, 

 where the lower conglomerates contain fragments of the syenitic rocks." 



Fourth. Dolomites, schistose or slaty micaceous beds, limestone, marble, 

 hydromica schists, and bastard marble. " This group of limestones and schists 

 rest on the above group, and are the equivalent of the Cambrian limestones of 

 the Great Valley." 



Fifth. Hydromica schists, quartzose schists, chloritic schists, and occasional 

 beds of quartzites and sandy beds and serpentines. 



Sixth. Micaceous, garnetiferous schists, limestone, mica schists and sand- 

 stones. " This group rests unconfoi'mably upon the western extension of the 

 second group." 



Seventh. " The mica schists of Philadelphia, mica schists, hornblendic, gar- 

 netiferous, talcose schists with soapstone and serpentine They rest 



unconformably upon the first, second, third and fourth groups There 



are besides these groups probably two serpentine horizons, which are undoubt- 

 edly unconformable deposits above the second group. I think the northern 

 belt of serpentine may be considered as altered Hudson river rock ; while the 

 southern belts are doubtful." 



The slates of Chester, York, and Lancaster Counties (including the 

 Peach Bottom slates), as well as the serpentines of Radnor, Easttown, 

 Willistown, and East and West Goshen, are stated to be undoubtedly 



