^2. NEW YORK STATE MUSEUM 



It is probably impossible to determine the structural relations of 

 the successive igneous invasions. It is difficult to conceive of the 

 method of approach which might give one of the later magmas 

 opportunity to invade superior rocks in so many places, especially 

 thoroughly crj'stalline and substantial rocks. That this happened, 

 however, is certain and a magma reservoir of very wide distribution 

 must be assumed. All these necessary assumptions are strengthened 

 by the conclusion reached in the discussion of correlation, which 

 indicates essentially identical igneous formations, and probably actual 

 connection between similar units, occurring in the Adirondacks on 

 the one side and in the Highlands of New Jersey on the other. There 

 is no doubt about the petrographic similarity of these types and the 

 only logical conclusion which seems to be warranted is that certain 

 of these magmatic masses must have had a remarkably widely dis- 

 tributed plutonic development. 



The simplest conception would seem to be a great bathylithic 

 mass extending beneath the whole region thus invaded which by its 

 successive manifestations of igneous activity produced the separa- 

 rate individual units of the series of this area. It may indeed be 

 that the different units are nothing else than successive developments 

 from this single larger plutonic source and that the historical range 

 represented does not transcend the limits of its long magmatic 

 history. In this case, products furnished by the distinguishable 

 field units must represent simply the manifestations of particular 

 periods of magmatic activity. It is difficult otherwise to conceive 

 of conditions which would produce so widely distributed results of 

 similar character with all of this peculiar complexity of structural 

 relation. A plutonic mass capable of producing the first effects, if 

 allowed to crystallize completely, could not, on subsequent igneous 

 invasion, yield the complex structures found so abundantly dis- 

 tributed in this territory. It would seem, therefore, that the funda- 

 mental conception, as the source of all the older igneous masses, is 

 that a single great regional bathylith, in its successive periods of 

 activity, has produced all these results as steps in a single but long 

 continued magmatic history. 



Deformation Structures 



These structures include unconformities, faults, folds and 

 crumples, crush zones, slaty cleavage, shear products, streaked habit 

 and schistosity. Some of these are of large geological significance, 

 such as the unconformities, folds and faults. Some of the other 



