32 NEW YORK STATE MUSEUM 



All the inclusions of anorthosite above mentioned bear exactly 

 the same relations to the inclosing syenite-granite as do the inclu- 

 sions of Grenville, and it seems clear that the upward moving 

 syenite-granite magma enveloped masses of both these rock series 

 in exactly the same manner. Thus we have just as strong evidence 

 that the syenite-granite is distinctly younger than the anorthosite 

 as that it is distinctly younger than the Grenville. 



Absence of Grenville and syenite-granite from the anorthosite 

 area. It is a striking fact that both Grenville and syenite-granite 

 are almost, if not quite, absent from a large part of the anorthosite 

 area of the Adirondacks. In the northeastern half of the anortho- 

 site area there are considerable developments of both Grenville 

 and syenite-granite. In the southwestern half of the Adirondack 

 anorthosite area, including the Schroon Lake quadrangle, the 

 absence of Grenville and syenite or granite is, however, an impres- 

 sive fact, though it must be remembered that many square miles of 

 this have not been carefully studied. The detailed Long Lake, 

 Schroon Lake and Paradox Lake maps, and the southern half of 

 the Elizabethtown map, show no Grenville or syenite-granite well 

 within the anorthosite there mapped. So far as known to the 

 writer this is also true of the southern half of the Mount Marcy 

 quadrangle. 



Bowen, in his paper on " The Problem of the Anorthosites," 

 dwells upon this absence of Grenville and syenite-granite from so 

 much of the anorthosite, 'and he offers an explanation briefly stated 

 as follows:^ "If one pictures the syenite and the anorthosite as 

 conventional batholiths, some difficulty is experienced in accounting 

 for the foregoing facts [see above paragraph]. It is necessary to 

 imagine an early intrusion of a huge plug of anorthosite followed 

 by an intrustion of syenite which took the form of a hollow cylinder 

 circumscribing it and invading it only peripherally. . . . On the 

 other hand, if one pictures the Adirondack complex as essentially a 

 sheetlike mass with syenite overlying anorthosite . . . one would 

 expect to find areas of Grenville roof covering the syenite in places 

 and to find it relatively little disturbed. In the interior and eastern 

 region of maximum uplift one would expect to find the deep- 

 seated anorthosite laid bare and to find it free from areas of the 

 roof." Also, he says, because of the deep erosion in the region of 

 maximum uplift one would expect to find the layer of syenite 

 removed. 



1 Jour. Geol., 25:223-24. 1917. 



