THE CALCITK VEINS. 261 



No other view of tliis long, narrow exposure seems reasonable than 

 that it is a vein, although Emmons cites it as a convincing argument for 

 the intrusive nature of limestone.* The limestone as now exposed to 

 observation is much longer and narrower than indicated l)y Professor 

 Emmons. 



Genetic Considerations. 



The problem of the original condition of these rocks, involving as it 

 does the changes through which they have passed and the respective 

 parts played by the local and regional metamorphism, is an obscure one 

 and perhaps in many points beyond solution, but its grander features 

 can be quite well understood. 



It is well appreciated by the writer thatlithologic similarity, especially 

 in metamorphic regions, is not a formof evidence to be implicitly trusted 

 as to the geologic unity of disconnected and scattered exposures. It can- 

 not be denied that these limestones and schists may belong to horizons 

 quite widely separated and that in a great series of gneisses and schists 

 there may be a number of calcareous horizons, but nevertheless the gen- 

 eral presence of these rocks of practically identical character over so wide 

 and closely related an area gives the observer a strong impression of their 

 original continuity, broadly speaking, and in this the writer feels in close 

 accord with the writings of James Hall, in 1876, previously cited. • That 

 there is any marked break to be detected between the underlying gneisses 

 and the limestone cannot be asserted. Apparently there is a continuous 

 series of strata especially rich in these rocks at one horizon and perhaps 

 more. Furthermore, although we have usually regarded our gneisses as 

 basal Laurentian it does not appear certain that we have in the Adiron- 

 dack region, as yet explored, any rocks older than the Grenville series 

 of Canada. The analogy with the Grenville is very close. 



The persistence and extent of the limestones and schists give ground 

 for thinking that over this portion of New York there Avas spread this 

 series of calcareous sediments, and that there were sandstones now repre- 

 sented by the quartzose mica and graphite-schists, and probably also 

 aluminous shales (now represented by the black hornblendic schists), 

 which, however, may be gabbroic intrusions. They rested on the origi- 

 nals of tlie gneisses which probably involved both mechanical sediments 

 and granite as well as other intrusions. The calcareous deposits were 

 not of great thickness and were sometimes highly magnesian and sili- 

 ceous, at other times aluminous. Whether they were to some extent meta- 

 morphosed before the vast plutonic intrusions of the Norian anorthosites 



* Report on the Second District, p. 39. 



