G. H. Williams — Peridotites near PeeksTcill, N~. Y. 27 



Dana of New Haven, who called his attention to these rocks 

 as affording promising material for petrographical study, and 

 kindly volunteered to guide him in an excursion over them in 

 the fall of 1883. A large number of specimens was subse- 

 quently collected ; but many other pressing duties have since 

 retarded the progress of the work. 



This group of rocks, covering an area of not over twenty- 

 five square miles, is composed of many and varied members. 

 It is separated quite sharply from the gneisses, mica-schists and 

 limestones which surround it, showing none of the gradual 

 transitions into these rocks which Hermann Credner, in his 

 description of this district written in 1865* supposed to exist. 

 Professor Dana, who encountered these rocks in the course of 

 his studies of the limestone belts of Westchester Co., JST. Y., 

 designated them as the " Cortlandt Series,"f from their being 

 principally confined to the township of Cortlandt, and pub- 

 lished a quite detailed account of their mode of occurrence 

 and macroscopic characters. He at first thought that they 

 might have resulted from the metamorphism of very ancient 

 volcanic ashes stratified by water while the surrounding sedi- 

 ments were being deposited.:]: He has, however, since ex- 

 pressed the opinion, based on several new and excellent expo- 

 sures, that at least the most basic members of the series are 

 truly exotic, intrusive m asses. § 



Professor Dana has divided all the massive Cortlandt rocks 

 into five classes according to the nature of their most import- 

 ant non-feldspathic ingredient,! viz : (A) Hornblendic (diorite), 

 (B) Hypersthenic (norite), (C) Augitic (diabase and gabbro), 

 (D) Biotitic (diorite) and (E) Chrysolitic (peridotite): This 

 classification may perhaps be advantageously followed, pro- 

 vided it be remembered that no sharp line can be drawn be- 

 tween the different groups ; but that, on the contrary, every 

 possible transition from each group into every other occurs. 

 Indeed I know of no other region of massive rocks so well 

 calculated to show the transitions, both sudden and gradual, 

 of one rock-type into another. 



The writer now proposes to describe petrographically the 

 different types of the Cortlandt Series in succession, commenc- 

 ing with the most basic ; this may be followed by an account 

 of the highly contorted and metamorphosed schists which 

 occur around their edge, while any general conclusions regard- 

 ing the origin and material of these rocks will be reserved to 

 the last. 



Such a study will serve as a contribution to our knowledge 



* Zeitschrift der deutschen geologischen Gesellsehaft, xvii, 1865, p. 390. 

 f This Journal, III, xx, p. 194, Sept., 1880. 



% Ibid., Ill, xxii, p. Ill, Aug., 1881. § Ibid., Ill, xxviii, p. 384, Nov., 1884. 

 |f Ibid., Ill, xx, p. 196, Sept., 1880. 



