26 THE JOURNAL OF GEOLOGY. 



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.' The eleolite-syenite area in northern New Jersey 

 is probably of the same character. 



In Pennsylvania and Maryland we find in the South Moun- 

 tain or Blue Ridge, between Harrisburg and the Potomac^ one 

 of the most highly diversified and perfectly preserved areas of 

 pr^-Cambrian volcanic rocks in the world. Its position is estab- 

 lished 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 devitri- 

 fication 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.- The 

 entire misunderstanding of these rocks by Rogers, Hunt, Lesley 

 and Fraser, who interpreted them as altered slates and their sec- 

 ondary cleavage as bedding, has greatly retarded the solution 



^ Professor Dana once suggested that the Cortlandt massive rocks might have 

 been formed by the metamorphism of "volcanic debris or cinders" (Am. Jour, of 

 Science, 3d ser., Vol. 22, p. 112, Aug. 1881), but he subsequently admitted their intru- 

 sive character (ib. Vol. 28, p. 384, Nov. 1884). See also opinions of the present writer 

 (ib. Vol. 36, p. 268, Oct. 1888). 



^Am. Jour, of Science (3rd ser.) Vol. 44, December, 1892, and Vol. 46, July, 1893. 



