4 i 8 T//E JO URN A L OF GEOLOG V. 



may well be due to unlike environment making deductions in 

 favor of unconformity to a certain extent misleading, but the 

 contrasts noticed are too strongly marked to admit of dispute 

 as to cause. 



The gneisses and schists of the older rocks are characterized 

 by a wide-spread development. Colorless muscovite, chlorite, 

 orthoclase, biotite, and quartz occur as essential constituents ; 

 epidote, zoisite, titanite, and garnets occur as accessories. Of 

 these, the first four minerals occur much more sparingly in the 

 upper series ; the last three are not remembered to occur at all. 

 Phases of the lower limestones carry tremolite or serpentine, while 

 dark hornblende occurs in abundance. Orthoclase is relatively 

 much less abundant in the border rocks where it occurs fre- 

 quently as pebbles. Pale-green, pleochroic muscovite, secondary 

 plagioclase, magnetite, and ottrelite, so common in the upper 

 series, are much less abundant in the lower series ; green-mus- 

 covite and ottrelite are not known to me in the central area. 

 The limestones of the two belts may also differ as to the per- 

 centage of carbonate of magnesium present. No investigation of 

 this subject has been attempted. 



Reference has already been made to the metamorphosed 

 basic igneous rocks, amphibolites, of the central area. One of 

 the best sections of these rocks is displayed in the railroad-cut 

 at Summit station, where they are exposed for nearly half a mile. 

 Numerous separate members can still be distinguished in the 

 mass by textural variations. They are cut by dikes of the same 

 material and also by more modern dikes of camptonite. Such 

 a series of amphibolites probably represents a period of volcanic 

 activity, antedating the Cambrian, of great areal extent. Nearly 

 everywhere, where these lower rocks are exposed, amphibolites 

 are present also. To the north they occur only in scattered 

 patches associated with granitoid gneiss ; to the south reconnais- 

 sance work has not detected them, but they probably occur 

 there. Mr. Wolff has described an amphibolite from a hill situ- 

 ated about one mile south of Mount Holly station, and he refers 



