136 NEW YORK STATE MUSEUM 



are so cut up and modified by igneous introduced substances that 

 their former sedimentary habit is elsewhere ahiiost completely 

 destroyed. 



Such selective effect involves an assumed control of injection by 

 the quality of the invaded rock, as the limestones are not heavily 

 injected or impregnated or even extensively silicated. This is true 

 even of the small interbedded limestones in the Fordham formation. 

 They are not usually badly enough affected to lose their identity in 

 spite of the fact that they lie in the midst of large igneous injections. 

 This can not be doubted for a moment if one has opportunity to 

 inspect the whole series. If such selective control is true for the 

 smaller members, it may very well be still more prominently 

 exhibited by the Inwood. If the small members succeed in reject- 

 ing the invading matters, such a large member as the Inwood might 

 very well escape without granitization at all and with only dikes and 

 pegmatite veins cutting at random through it just as the formation 

 now stands. It would seem to be possible, in other words, that 

 members below the Lowerre might be so thoroughly invaded as to 

 make all the complex gneiss structure that we have in New York 

 City, while at the same time the overlying two members which give 

 less encouragement to injection, might be as little transformed as the 

 Inwood and Manhattan actually are. It is a most striking thing in 

 this connection that the Manhattan schist is many times more 

 affected by igneous matters than is the limestone which lies immedi- 

 ately below. Doubtless selective action of this kind is an important 

 matter in the general process of injection and especially in that phase 

 of it referred to as impregnation. 



The striking thing is the evidence that seems to be furnished 

 pointing to the possible Grenville age of the Manhattan-Inwood- 

 Lowerre-Fordham series. In other words, if the Fordham is 

 Grenville and the Inwood is conformable with and a continuation of 

 it, and if the Manhattan is a normal succession after the Inwood, 

 then there seems to be no escape from the conclusion that this whole 

 series is Grenville and of Precambrian age. 



It is most unusual that there should be doubt as to whether a 

 formation is Grenville or Cambro-Ordovician, but there is such a 

 doubt in this case. 



6 A sixth point in support of the Grenville age of the southerly 

 series is the fact that in two of the very best exposures, that under 

 the Harlem river and the other under Delancey street, an interbedded 

 layer of quartzose gneiss was encountered at some distance above the 

 base of the Inwood. In one case, at Delancey street, the bed is only 



