_y 



472 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



sandstone and Magnesian limestone^ conformable with Loth, and that 

 they theref(^rc cannot belong to the " Taconian" of Iliintj as the latter 

 had stated. Professor Prime also says : — 



*' It is well here to emphasize the fact tliat these brown hemotit(3 ores all 

 Leh>T)g to the Lower Silurian limestone formation, siucCj in 1875, Dr. Slerry 

 IlLiiit, alter a eui-sory examination of Ziegler's Mine in JVA^ County . . . . 

 made the mistake in a paper on ' The Decay oi" Crystalluie Eocks ' Ijeiore the 

 National Academy of Science, of supposing that the hydrouiica slates belonged 

 to the Hnnniian Period: — a mistake hito which so eminent an observer as 

 himself would never have fallen had he been better acquahited Aviih the 

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

 1878, (3) XV., pp. 2C1-2G9.) 



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

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

 pp. 435-443) : 



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



rocks. 

 Second. A series of syenitic, hornblendic and (|uartzose rocks. Ho states, 



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



Third. Potsdam sandstone, conghnnerate, (piartzite, and occasional ' schis- 

 tose beds. " This, the Potsdam sandstone, rests imconformably upon the pre- 

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

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



Fourth. D(jl(jmites, schistose or slaty micaceous beds, limestone, marlde, 

 hydromioa schists, and bastard marble. " This group of limestones and scliists 

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



the Great Valley." 



Fi/lh. Ilydromica schists, quartzose schists, chloritic schists, and occasional 



beds of quartzites and sandy beds and serpentiiies. 



Sixlh. Micaceous; garnetiferous schists, limestone, mica schists and sand- 

 stones. " Tliis group rests unconformably upon the western extension of the 



second, group." 



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

 netiferous, talcoso 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 ; Avhile 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 



