GEOLOGY OF THE NORTHERN ADIRONDACK REGION 299 



summit to the series is therefore what would naturally be ex- 

 pected, and it may be legitimately argued that the thickness must 

 have been very great, since so great an amount of rock has be6n 

 worn away. It is by no means meant to imply that these rocks 

 formed the whole mass of what has been removed, but it is 

 thought that they must have constituted a respectable percentage 

 of it. Even the remaining fragments indicate a very considerable 

 thickness for the formation. 



The nondiscovery of the base is not so easily accounted for. 

 It is a water-deposited formation and must have been laid down 

 on some floor, and it would naturally be expected that some 

 evidence of what this floor was would be forthcoming. But the 

 great metamorphism which has destroyed the old rock structures 

 and given them a common foliation, the inextricable intermingling 

 of igneous rocks with the Grenville sediments, and the later 

 great igneous invasions from beneath have so disguised the rock 

 relationships as to make it very likely that the base of the Gren- 

 ville will never be satisfactorily made out in the region. 



Doubtful gneisses (" Saranac " formation) . In the portions of 

 the Adirondack region with which the writer is familiar the only 

 large body of gneiss which is practically free from all Grenville 

 admixture and at the same time seems to have no connection 

 with the later igneous intrusions, is found in a belt running 

 through northern Clinton and Franklin counties, adjoining the 

 Potsdam boundary. It is not utterly free from Grenville rocks, 

 since a few small patches of these do occur, though unfortunately 

 exposures which disclose the relations between the two nowhere 

 appear. The presence of these few small patches in the great 

 body of gneiss furnishes one of the main arguments for the dis- 

 tinction between the two, since it is unlikely that the distinctive 

 Grenville rocks would be present in such slight quantity were the 

 gneisses affiliated with them, that is, were either sediments or 

 were igneous rocks of Grenville age. 



These gneisses are prevailingly red, acid gneisses whose usual 

 feldspar is orthoclase (or microperthite or microcline), and which 



