36 NEW YORK STATE MUSEUM 



bers the gradations and variations are more strikingly represented, 

 but their origin as magmatic differentiates is not so certain. For 

 example, the more vigorously penetrating members and those which 

 were intruded during movement, as well as those which had strong 

 pegmatitic tendencies, have a tendency to develop extreme differ- 

 ences in very short distances giving a structural habit more than a 

 differentiation effect. Wherever the structure is massive or uniform, 

 variation is also simple and gradual, but in portions with strong 

 gneissoid structure or strong pegmatitic tendency, the differences 

 are more abrupt and the extremes are much greater. Judging from 

 the structures represented, it appears that deformation during certain 

 stages of crystallization is not only an aid to differentiation, but a 

 very prolific cause of pronounced complicated and variable structure. 

 It is the writers' belief that the movement represented is largely 

 regional deformation rather than internal convection within the mag- 

 matic mass and that some of the Highlands gneisses owe their chief 

 structural habit to this process. T|hey are therefore apparently not 

 simple gneissoid granites but are in part dynamic and to that degree 

 have many of the characteristics of true gneisses. That they have 

 not been deformed or recrystallized to any considerable amount 

 since complete solidification is practically certain, and it is absolutely 

 certain that their chief structure is not due to recrystallization at all. 

 To this degree, therefore, the rocks of this structural habit and rela- 

 tion are more properly representatives of the gneissoid granites than 

 of true gneisses. 



It should not be concluded from the emphasis placed on this 

 process, however, that we consider this the method by which all 

 the streaked and banded rocks were made. As a matter of fact, 

 it seems to us that most of them have developed under quite different 

 conditions and influences, with the aid of syntexis, injection, impreg- 

 nation and structural control from the previously existing forma- 

 tions. In other words, it seems to us that banded and other strong 

 structures of the same general habit are in large part antecedent 

 structures imposed upon the igneous mass and preserved in it even 

 where the original mineralogy may have been destroyed. 



It is scarcely within the province of such a paper as this to dis- 

 cuss the mechanics of differentiation and the different theories of 

 the working of this principle, and we do not undertake to argue 

 for or against the principle of liquation, or the separation of mag- 

 matic liquids of different quality, as compared with the principle of 

 fractional crystallization or the separation and settling of crystals in 



