2^2 NEW YORK STATE MUSEUM 



The Precambric rocks are classifiable in order of age from the 

 latest to the oldest as follows: 



The iinmetamorphosed basaltic dikes 



The eruptive complex of more or less metamorphosed granites, 

 anorthosites, syenites, gabbros and intermediate types. 



The Grenville series of limestones, ophicalcites, schists, and sedi- 

 mentary gneisses 



Grenville series. The name Grenville was originally given by 

 Logan to a series of rocks in all respects similar to the one here 

 under discussion and developed in the township of Grenville, On- 

 tario. Ebenezer Emmons in his early work in the Adirondack area 

 spoke of them as primary, and, under this head, placed the lime- 

 stones (called primitive limestone) and the serpentine with the ig- 

 neous rocks, while the gneiss was classed with the stratified. It is 

 one of the curious instances of the changes in geological thought, 

 that 60 years later these views are exactly transposed. In later 

 years the wise custom has developed of applying geographical 

 names to formations and for this reason the term Grenville is here 

 adopted. It is true that a gap intervenes between the Adirondacks 

 and the Canadian exposures in Quebec and Ontario, and that this 

 gap is covered by the Paleozoics, but the similarity of the old sedi- 

 ment's in both areas is so great, that there seems little doubt that 

 they are equivalents. The International committee, which visited 

 both regions in 1906 and submitted a report on the correlation of the 

 two were at least sufficiently impressed with the similarity to 

 recommend the uniform use of Grenville.^ 



The Grenville strata are widespread in the Adirondacks, scarcely 

 a quadrangle being without them. On the Port Henry sheet and 

 along Lake Champlain just north of Port Henry is one of the best 

 exposures in the eastern mountains, but they also appear at a 

 number of other localities in the area here described. 



The most prominent and easily recognized of the members is a 

 white crystalline limestone, very coarse grained and seldom pure 

 or uniform over any great width. It is marked by small inclusions 

 of p3^roxene, graphite and less common individual minerals and 

 by larger streaks and pegmatitic aggregates of coarse quartz, feld- 

 spar, hornblende, biotite, tourmalin, titanite, pyrrhotite and scapo- 

 lite. Where the limestone has been quarried for fluxing purposes 

 in the iron furnaces, as has been the case near Port Henry, large 

 dumps of the rejected silicates have accumulated, and now afford 



1 Jcur. Geol. 1907. 15:191. 



