30 NEW YORK STATE MUSEUM 



the limits and exact nature of the other members are very uncertain 

 indeed. 



The igneous representatives are the most troublesome of all, not 

 because of the difficulty in distinguishing between the members, 

 but because, if one is interested in the genesis of their variations, 

 it is necessary to consider so many possibilities of origin. Because 

 of the obscurity of some of these conceptions, it is thought best to 

 discuss this matter at some length before the individual formations 

 are described. 



Causes of variation in the crystalline rocks. The chief causes 

 of variation may be listed as follows : 



1 Original differences of composition in sediments. 



2 Dynamic Metamorphism with recrystallization and deformation. 



3 Original magiuatic differences of the invading igneous magmas. 



4 Magmatic differentiation within these igneous masses. 



5 Magmatic movement with development of gneissoid habit. 



6 Syntexis or magmatic absorption. 



7 Igneous injection in its many forms of lit-par-lit banding, peg- 

 matitic bunches, veins and dikes, 



8 Igneous impregnation penetrating the grain of the original rock 

 by extremely fluid and vigorous igneous matters, resulting finally in 

 a mixed product which is in part original and in part introduced 

 materials. These two causes (7 and 8) together give every possible 

 gradation from one to the other and from both to the original 

 invaded rock on the one side and to the invading igneous rock on 

 the other. 



9 Contact effects produced by igneous rocks on older rocks of 

 all kinds. 



10 Deformation which has resulted in the usual shearing, granula- 

 tion, crumpling, recrystallization and schistosity. 



These are the usual causes of variation in ancient rocks of com- 

 plex history, but it seldom happens that all of them are so exten- 

 sively developed and displayed so magnificently as in the Highlands. 

 After one has studied the region and become convinced of the major 

 features of its geology, the formations, with all their apparent con- 

 fusion, do not appear a hopeless mess, but show clearly the effects 

 of a most interesting series of processes, ari understanding of which 

 adds immeasurably to their interest. Instead of being a confused 

 jumble, they constitute one of the best exhibits of deep-seated, 

 vigorous transformation processes to be found within easy reach of 

 large centers of population within the borders of the United States. 



