48 NEW YORK STATE MUSEUM 



e Igneous rocks. These occur in great variety, and represent 

 either plutonic or large intrusive masses or small dikes or veins. No 

 surface flows are represented in this field. 



The acid type includes granite, alaskite and aplite. 



The intermediate type includes syenite and syenite porphyry. 



The medium basic rocks include diorite, norite, quartz-diorite and 

 camptonite. 



The more basic group includes gabbro norite, pyroxenite and 

 dunite. 



Each principal type of rock is represented in the field by numerous 

 facies covering a wide range of mineral proportion and habit. The 

 actual number of varieties would be very great. 



The most variable differentiates are pegmatites varying from sim- 

 ple quartz pegmatite to hornblende rock and magnetite ores. Some 

 of these occur as veins, some as dikes and some as injections and 

 impregnations. 



/ Mixed types. Mixed types are in part listed under d, but they 

 include certain ones not mentioned there, especially the syntectics, 

 which may appear as granite gneisses or gneissoid granites, the 

 injection gneisses which are usually strongly banded, and the sili- 

 cated metamorphics, chiefly represented by the Grenville series and 

 xenolithic masses. 



The Mappable Formations with their Petrography 



The formations discussed under this heading occur in large enough 

 development to require representation on the areal map. The chief 

 bases of delimitation are: (a) unity of origin, (&) like structural 

 or field relations, and (c) petrographic constancy. But in some 

 cases the petrographic variety is so great within a single unit that 

 this last principle of identification alone would not suffice. 



On this account, the mapping of the region is difficult. The for- 

 mations themselves have great obscurity and exceedingly great com- 

 plexity as indicated in the preceding discussion; these conditions 

 make the problem of dividing them into definite units for mapping, 

 more or less uncertain. The result is a generalization, necessitated 

 by the limits imposed by the map itself. Small differences can not 

 be mapped on a moderate scale ; only the large divisions considered 

 of primary importance can be taken into account and indicated. 



It has been necessary, therefore, to study the region with the 

 object in mind of determining those characteristics indicative of 

 the history of each formation and to apply them as well as possible 



