88 ROCK-METAMORPHISM. 



New York, and New Brunswick, considered by many geologists to 

 be of post- Archaean age, as the altered Quebecs, Emmons's " pri- 

 mary limestones/' &c. A corresponding formation (the lime- 

 stone of Essex and adjacent counties) is considered by Pro- 

 fessor James Hall to belong to a " period subsequent to the 

 deposition, metamorphism, and disturbance of the rocks of" 

 authentic Laurentian age," and to apparently hold a place in 

 the series between the Laurentian and the " Potsdam periods ; 

 but whether of Huronian age or otherwise " he does " not pre- 

 tend to say ; and it may even prove of later date than this"*. 



Contending for the existence of true Archseans or Lauren- 

 tians in the highland region of Northern New York, and 

 that these rocks are unconformably overlain by a younger and 

 well-developed series of gneisses, mica- and hornblende-schists 

 (including crystalline limestones and ophites) in Westchester and 

 other adjoining counties, Prof. Dana has of late endeavoured to 

 prove, by their relations to certain fossiliferous deposits occur- 

 ring in neighbouring places, that the younger metamorphics 

 are of Upper Cambrian and Lower Silurian (" Calcif erous," 

 " Quebec," " Chazy," and "Trenton") agesf. Should this 

 prove to be correct, there will be no need of trying to make 

 out that the crystalline limestones they contain were ever other- 

 wise than calcareous ; for as such they may have been simply 

 mineralized. Nevertheless it does not seem improbable that 

 some of the Westchester crystalline limestones may be methy- 

 lotic products, particularly as there is no reason to exclude 

 from the series containing them two or more older groups, 

 the Acadian and Potsdam, whose remarkable deficiency of cal- 

 careous matter in their unaltered condition would render it 

 highly probable, should they in their metamorphosed state 

 contain well- developed crystalline limestones and ophites, that 

 metamorphosing agencies had generated such limestones. 



* ' American Journal of Science and Arts/ 3rd series, vol. xii. p. 300. 



t American Journal of Science [and Arts,' 3rd series, vols. xix. and xx., 

 "Geological Relations of the Limestone Belts of Westchester Co., N. Y." 

 The reader is referred, in this connexion, to Selwyn in ' Canadian Naturalist,' 

 new series, vol. ix. pp. 17-31. 



