464 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



upper." He concludes by suggesting the inquiry " whether the Eozoon 

 limestones of Canada, which are associated with Laurentian rocks, and 

 have been referred to that age, are I'eally Laurentian." 



Professor Hall's idea of making a separate geological formation or 

 system out of the limestone masses of Northern New York, on account 

 of their unconformable position, is hardly less remarkable than that of 

 Professor Emmons, who considered that the peculiar occurrence of this 

 rock in the region in question could not be accounted for except on the 

 supposition that they were of eruptive origin. It seems to us, on the 

 other hand, that these limestone occurrences are, very probably, similar 

 in character to those of Eastern Massachusetts, which are not a part of 

 the stratified formation, but rather of the nature of segregated masses, 

 or chemical precipitates, as will be more fully set forth farther on in 

 this paper. 



Prof. A. R. Leeds, in a paper entitled " Notes upon the Lithology of 

 the Adirondacks," concluded 



" that the rocks of Essex County are part of the Norian System," and " that 

 these norites are a stratified rock, but have undergone a metamorphosis so 

 profound as to have caused them to lie regarded by Prof. Emmons and eaiher 

 observers as unstratified. The dolerites which are found of the same con- 

 stituent minerals, and are of the mean specific gravity of these norites, have 

 probably been formed from a portion of these stratified deposits by deeply 

 seated metaniorpbic action, and have further modified and greatly tilted the 

 superposed rocks in the course (jf their extrusion." (American Chemist, 1877, 

 VIL, p. 339.) 



That these rocks belong to the Norian system was determined by 

 lithological evidence, which really means no more than this, that gab- 

 bros coming from different localities look somewhat alike. That the 

 rocks are stratified is, according to him, shown by the existence of a 

 more or less complete parallel arrangement of the constituent minerals. 

 Professor Leeds is a chemist, and not a petrographer ; and since his 

 ideas are obviously simply a repetition of those of Dr. Hunt, his testi- 

 mony neither adds to nor detracts from the importance of the theoreti- 

 cal views of the latter. 



Mr. N. L. Britton, in 1881, in an article on the geology of Richmond 

 County, N. Y., (Annals of the N. Y. Academy of Sciences, 1881, IL, 

 pp. lGl-182,) advocates the view that the granite observed by him at 

 Tompkinsville is an Ai'chsean metamorphosed rock. This granite is 

 said by him to be "very coarsely crystalline in structure," and no strati- 

 fication is observable in it. No proof is furnished that it is of meta- 

 morphic origin. 



