GEOLOGY OF THE NORTHERN ADIRONDACK REGION 343 



one can seldom be sure in such cases that one is not dealing with 

 a later intrusive. 



That there are igneous gneisses in the region which are older 

 than the anorthosite is certain, since such rocks are found cut 

 intrusively by it. That these rocks are for the most part 

 thoroughly gneissoid, more so than is true of any of the larger 

 intrusive masses, is also certain. It would therefore seem that 

 they must have been somewhat metamorphosed before the appear- 

 ance of the intrusives, but that the criterion is not one which can 

 be used in all cases for the purpose of discriminating between the 

 two sets of rocks. 



If the intrusion of the great igneous masses had been the prime 

 factor in the metamorphism of the older gneisses, their foliation 

 should show a general parallelism to the boundaries of the in- 

 trusions. In general it does not show this, but on the contrary 

 is mostly independent in direction. Adams has described a 

 notable instance in Canada, where the strike of the foliation of 

 the gneiss around the Morin anorthosite rather minutely parallels 

 the boundary on three sides of the mass. 1 But it does not follow 

 it on the fourth side, the anorthosite is also foliated near the 

 boundary, and its foliation everywhere parallels that of the 

 gneiss, and Adams regards it as having been produced in both at 

 the same time, and necessarily subsequent to the intrusion. 



No similar case of striking parallelism has been noted in the 

 Adirondacks so far as the writer is aware, and it is also true here 

 that locality for locality, the foliation of the older gneisses and 

 of the intrusions corresponds, indicating that it is due to a com- 

 mon cause, operating after the appearance of all the intrusives, 

 since they all show foliation. It is no doubt true, as urged by 

 Adams, that the contact lines between the two sets of rocks will 

 form lines of weakness, along which there will be a special 

 tendency to stretching, and which may locally influence the direc- 

 tion taken by the foliation, when not overbalanced by other 

 things. But so far as the writer's observation in the Adiron- 

 dacks goes, parallelism is the exception rather than the rule. 



The universal concordance in foliation between the gneisses 

 and the intrusives makes it impossible to say whether the former 

 possessed any previous foliation or not. If so, it was either 



HJeol. Sur. Can. 8 :13J-15J. 



