Page:The Journal of geology (IA journalofgeology21894univers).pdf/40

highly differentiated "Cortlandt Series," near Peekskill, presents us with the deeply eroded roots of an ancient volcano, probably of Cambrian or Silurian age, whose superficial parts have entirely disappeared.^[1] The eleolite-syenite area in northern New Jersey is probably of the same character.

In Pennsylvania and Maryland we find in the South Mountain or Blue Ridge, between Harrisburg and the Potomac, one of the most highly diversified and perfectly preserved areas of pre-Cambrian volcanic rocks in the world. Its position is established as below the Olenellus sandstone; it presents both acid (rhyolitic) and basic (basaltic) types; it exhibits within limited shear-zones the plainest effects of dynamic action, but its great mass is nevertheless so little changed that each microscopic structure of glassy rocks is clearly recognizable. Skeleton crystals, minute pores and larger vesicles, protoclastic breaking of the phenocrysts, fluidal structures of every kind, trichites, spherulites, axiolites, lithophysal and perlitic parting have lost none of their original sharpness, in spite of the complete devitrification of the glassy base. Most of the rocks were probably always wholly or mostly crystalline, but some regions, like the Bigham Copper and Raccoon Creek, display the old spherulitic obsidians and pumice in a manner allowing of no doubt. The pyroclastic materials accompanying these old lavas are also finely developed—ash-beds, coarse and fine flow- and tuff-breccias, etc. The precise centers of eruption within this region have not yet been definitely located, but with what has already been published regarding these rocks and the further details which may be soon expected, no further description of them is here necessary.^[2] The entire misunderstanding of these rocks by Rogers, Hunt, Lesley and Fraser, who interpreted them as altered slates and their secondary cleavage as bedding, has greatly retarded the solution