Page:The Journal of geology (IA journalofgeology21894univers).pdf/434

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 frequently 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-muscovite and ottrelite are not known to me in the central area. The limestones of the two belts may also differ as to the percentage 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 reconnaissance work has not detected them, but they probably occur there. Mr. Wolff has described an amphibolite from a hill situated about one mile south of Mount Holly station, and he refers