Page:The Journal of geology (IA journalofgeology21894univers).pdf/432
in grouping of the component minerals rather than to differences of composition. Gneisses are most common, occurring as fine-grained, chloritic rocks or coarse biotite, augen-gneisses. A brownish coarse gneiss with porphyritic crystals of orthoclase extends in intermittent outcrops from Wilcox Hill on the north to Button Hill on the south, a distance of eight miles. This rock carries both biotite and muscovite, the latter evidently derived from the feldspar. In Eastham, Northam, and east of Bear Mountain, there are areas of coarse biotite gneiss with interstratified beds of quartzite and limestone. Fine-grained, chloritic schists and gneisses are abundant, as on the summit of Saltash Mountain.
The area immediately about Mount Holly village on the Central Vermont Railroad, is characterized by a great number of amphibolites. These occur as schists, either intrusive or extrusive, and as dikes, cutting one another, and the country rock. They occur interlaminated with various rocks—quartzites, gneisses and schists, and possess the local schistosity of the enclosing rock. This is as true of the dikes as of the sills, affording a conception of how far removed from any key to the real stratification is the lamination of these rocks and how faulty geological interpretation must be when deciphered on the basis of induced structures. Aside from the interest one naturally feels in eruptives as old as these, their importance as evidence in separating the Mendon from the Mount Holly series cannot be overestimated. Modern basic dikes of camptonite and other igneous rocks traverse the core rocks, but they are younger than the last disturbance of the Green Mountains, cutting Algonkian and Cambrian rocks alike.
Following the accepted definition of the Algonkian rocks, this lower series as well as the upper must be grouped as Algonkian. Although possessing many rocks undoubtedly igneous, and others whose origin is problematical, there is a considerable development of genuine sedimentary rocks, warranting us to place the whole series among the Algonkian. The evidence for this sub-division, which is based upon manifold differences between