40 NEW YORK STATE MUSEUM 



processes might be repeatedly set in operation by succeeding mag- 

 matic invasions. In such cases a rock would become exceedingly 

 complex and even an ordinarv museum specimen might represent 

 not only the original country rock that preceded all the injections 

 but additional material from each succeeding igneous invasion. 

 Occasionally these individual port ions are in large enough bands or 

 are striking enough in their petrography to be correlated with their 

 source. But in many cases it undoubtedly happens that differentia- 

 tion of the invading fluid and its contamination by absorbed material 

 together with recrystallization of some constituents have given a rock 

 which is not readily assigned to a particular type. Clear cases, 

 however, are so frequent that there is no doub' of the importance 

 of the injection process in the manufacture of the banded gneisses 

 of the Highlands. It is our belief that the strongly banded gneisses 

 are more frequently of this origin than of any other simple origin, 

 that next to this process in the matter of competence to develop a 

 banded structure is syntexis, and that differentiation ])roper is the 

 least efficient of all. Injection phenomena are not difficult to under- 

 stand and in many cases are readily recognized, but in many of the 

 compHcated gneisses of the Highlands the confusion is so great 

 that it is impossible to indicate with certainty which structures are 

 due to one type of origin rather than another. It is a process the 

 understanding of which helps very materially toward an accurate 

 working conception of the meaning of the geology of the Highlands, 

 but in detail, in a minor way, the complete statement for every out- 

 crop is absolutely impossible. 



Igneous injection is related to magmatic differentiation, to syntexis, 

 to magmatic movement and td igneous impregnation and contact 

 effects. These are as a matter of fact only the different expressions 

 of the whole complicated process of deep-seated rock-making during 

 igneous invasion. 



All the ancient granite magmas have developed injection 

 phenomena, but it is more noticeable with the Canada Hill type, 

 the older diorites. the Reservoir type and the Storm King. The 

 Peekskill-Mohegan graniie and the Cortlandt norite series have had 

 little injecting power. 



Igneous impregnation. We are using this term to distinguish 

 insidious interpenetration of magmatic matters from the simpler 

 injection type just described where, as a rule, the introduced material 

 is distinguishable from the country rock. In true impregnation the 

 material that is introduced is capable of entering the interstitial 



