dfACONIC QUESTION IN GE0L0t4Y. 14S 



crystalline rocks iu sovith-easteru New York, as he would have his readers infer. When, 

 in 1858, H. D. Rogers had occasion, in his final Eeport on the Geology of Pennsylvania, to 

 describe the continuation of these same rocks into that State, he distinctly assigned them 

 to a horizon below the base of his paleozoic series, proposing, at the same time, a Hypozoic 

 and an Azoic system to include them. 



§ 186. The Highland range on the east side of the Hudson traverses Putnam county, 

 and, passing south-westward to the river, occupies but a small area in the north-west cor- 

 ner of Westchester County. Along its south-east base, at Annsville and at Oregon, is met a 

 narrow belt of scarcely crystalline limestone, accompanied by an argillite or talcoid slate, 

 and resting unconformably upon the ancient gneiss. This belt, apparently a Lower Ta- 

 conic outlier, is regarded by Dana as partially altered Lower Silurian, and "the grade of 

 metamorphism " is declared by him to become more intense to the south and east, giving 

 rise to the whole gneissic area of Westchester and New York Counties. The gneisses and 

 conformably interstratified crystalline limestones of this large area are, as we have seen, 

 supposed by Dana to be metamorphosed Lower Silurian, though they are really undis- 

 tinguishable from the rocks of the adjacent Highland range, which he admits to be Archean 

 or Primary. In support of his startling proposition, Dana might be expected to point ou.t 

 some distinctions between the rocks of the two areas. He begins by suggesting certain 

 differences as to more or less micaceous or horublendic gneisses in the two regions in ques- 

 tion, but confesses that " there are gradations between the two, in both respects, which 

 make the application of a lithological test very perplexing," and admits that "the litholog- 

 ical evidence of diversity of age is weak," "' a criticism which the intelligent reader will 

 conclude is equally applicable to Dana's stratigraphical argument. I am familiar with 

 the rocks of many parts of Westchester County, and since the publication of Dana's 

 paper in 1880 have taken repeated opportunities to examine, in various localities, the rocks 

 called by him Metamorphic Lower Silurian, as at Singsing, Tarrytown, Yonkers, Spuyten 

 DuyAàl and Kiugsbridge, along the Hudson. I have also stvidied the same rocks farther to 

 the east, along the River Bronx and the Harlem Railroad to Pleasantvale, as well as between 

 this line and the Hudson, and have crossed eastward to Long Island Sovmd and examined 

 the exposures on the shore at and near New Rochelle. Being already familiar with the 

 Laurentian rocks throughout Canada, as well as in parts of the Adirondacks, and in the 

 Highlands from Putnam County, New York, through New Jersey and Pennsylvania to the 

 Schuylkill and beyond, I do not hesitate to say that these gneisses and their associated 

 crystalline limestones of Dana's so-called Metamorphic Lower Silurian, in Westchester 

 County, cannot be distinguished from the typical Laurentian. I believe that the judgment 

 of an impartial observer would be that the notion of any difference between the Latiren- 

 tian gneisses and limestones of the areas mentioned, and the gneisses and their interstrat- 

 ified limestones of Westchester County, has no foundation in fact. 



§ 18*1. Passing now from Westchester County to the adjacent Manhattan Island, the 

 same Laurentian gneiss is seen in its northern portion, between Seventh and Eighth 

 Avenues, especially in a cutting at One Hundred and Forty-fifth Street, and thence in a 

 ridge some distance farther south, the strata being nearly vertical and of grayish horn- 



-' Amer. Jour. Science, 1880, xx, 373. 



Sec. IV., 1884. 19. 



