12 NEW YORK STATE MUSEUM 



GENERAL COXSIDERATIOXS 



The Precanibrian rocks as a group display a striking simplicity 

 and narrowness in their mineralogical variations which render 

 the individual formations easily recognizable in the field. This 

 is unfortunately not always the case with rocks of such extreme 

 antiquity, even in neighboring areas of like age; but withm the 

 district at present under consideration, such lack of ambiguity 

 concerning the different lithologic units is of distinct aid in unravel- 

 ing the mutual structural relations of the various rock types en- 

 countered. 



The oldest rocks of the region are undoubtedly those metamorphic 

 sediments which are referable to the Grenville series ; this correlation 

 is based on their similarity to those of the original Grenville area, 

 as regards their physical constitution and their structural and tec- 

 tonic relationships. Professor Gushing, while working along the 

 north border of the Adirondacks, thought it not impossible (1905, 

 page 274) that certain obscure gneisses whose relations to the 

 established Grenville sediments could not at the time be satisfactorily 

 made out, might represent the floor upon which these elastics were 

 deposited. This hypothesis should of course be entertained where 

 definite evidence of the contemporary or subsequent age of such 

 gneisses is not obtainable ; the discovery of the sub-Grenville base- 

 ment somewhere in the Adirondacks is not in itself an impossibility 

 from a purely theoretical point of view. But within the limits of 

 the Canton sheet, those formations which are not an integral part 

 of the Grenville series, for example granite gneiss and gabbro-am- 

 phibolite, are seen to exhibit distinctly intrusive relations with, and 

 to be therefore younger than, the Grenville, the basic intrusive 

 being the earlier of the two. If. therefore, the bedrock formations 

 of the Grenville sea bottom are present within the geographic limits 

 of the present study, they have been so altered through recrystalli- 

 zation in depth as to present the aspect of intru:=ions into the 

 superjacent sediments. Xo data bearing on the solution of this 

 difficult problem could be secured. 



The granite gneiss is of the type occurring extensively in adjoin- 

 ing Precambrian areas, which has been referred to by authors as 

 Laurentian (Gushing et al., 1910, page 10). Owing, however, to 

 the present tendency among American geologists to restrict the 

 term Laurentian to intrusives of post-Kewatin but of known pre- 

 Huronian age, and in view^ of the impossibility of obtaining in this 

 quadrangle any data bearing directly upon the problem of the corre- 



