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NEW YORK STATE MUSEUM 



With the passage of the rock into anorthosite gabbro, recrystal- 

 lization always comes much more largely into play, because of 

 the more varied mineral composition, facilitating corrosion and 

 causing more or less foliation. It is thought that there is here 

 a reasonable and likely the true explanation of the apparent 

 less metamorphosed condition of the anorthosite. 



The Grrenville and Saranac rocks are apparently more thor- 

 oughly metamorphosed than the later intrusions. They are 

 more uniformly granular, better foliated, much more completely 

 recrystallized and with a usual utter lack of all traces of origi- 

 nal textures. These same characters are also found in the in- 

 clusions of these rocks which occur in the intrusives and seem 

 to the writer to indicate that they were somewhat metamor- 

 phosed, at least, before the time of the intrusions. While no 

 doubt the heat and pressure incident on their intrusion must 

 have exerted considerable effect on the older rocks, the evidence 

 does not point to this as of prime importance in their meta- 

 morphism. 



But the distinction between the two sets of rocks is in many 

 ways not a sharp one and is difficult to apply. The more thorough 

 foliation and complete recrystallization of the Grenville rocks 

 may be accounted for by the fact that they were originally fine 

 grained sedimentary rocks, and that their metamorphism is not 

 necessarily more extreme than that of the intrusives. But the 

 more metamorphosed character of the igneous Saranac rocks can 

 not thus be accounted for. The whole problem of their age and 

 relationships is one of such uncertainty however that it is some- 

 what unsafe to emphasize comparisons between them and the 

 intrusives. They are cut in places by small masses of the in- 

 trusives and they must occur as inclusions in them. The diffi- 

 culty of the whole matter arises from the fact that the Sara- 

 nac rocks are so similar to the more gneissoid phases of the 

 granite, syenite and gabbro masses that it is frequently im- 

 possible to tell with which rock one is dealing. Small, later 

 intrusive masses in the Dannemora rocks may be so thoroughly 

 gneissoid as to appear like an integral part of the group. 

 Specially among the granitic Saranac gneisses traces of cata- 

 clastic structure are often found, and of igneous textures; yet 



