452 W. J, MILLER ADIRONDACK AXORTHOSITE 



Bowen*° says: "Xot only is the anorthosite unbroken by areas of Gren- 

 ville, especially away from the margins, but it is likewise practically free 

 from protrusions of the syenite.'' Although this statement needs to be 

 modified, I believe it constitutes the strongest single argument in favor 

 of Bowen's hypothesis. It is not, however, opposed to my conception of 

 the structure and origin of the anorthosite. The statement is by no means 

 true for the northeastern half of the anorthosite area which is covered by 

 the Lake Placid and Ausable quadrangles and the northern portions of 

 the Mount Marcy, Elizabethtown, and Port Henry quadrangles and the 

 western half of the Willsboro quadrangle. There are extensive develop- 

 ments of both Grenville and syenite-granite throughout this northeastern 

 half of the anorthosite area. In the southwestern haK of the area, how- 

 ever, the absence of Grenville and syenite or granite is indeed an impress- 

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

 have never been carefully surveyed. The detailed Long Lake, Schroon 

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

 map show no areas of Grenville or syenite-granite. As I understand it, 

 this is also true of the southern half of the Mount Marcy quadrangle. 

 According to Bo wen :*® 



"If one pictures the syenite and tlie anorthosite as conventional batholiths, 

 some difficulty is experienced in accounting for the foregoing facts. It is 

 necessary to imagine an early intrusion of a huge plug of anorthosite followed 

 by an intrusion of syenite which took the form of a hollow cylinder circum- 

 scribing 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 Gren- 

 ville roof covering the syenite in places and to find it relatively little dis- 

 turbed. In the interior and eastern region of maximum uplift one would ex- 

 pect to find the deep-seated anorthosite laid bare and to find it free from areas 

 of the roof." 



Also, because of the deep erosion in the region of maximum uplift, one 

 would expect to find the layer of syenite removed. 



My viev^ is quite different. As I have repeatedly shown in this paper, 

 it is certain that the anorthosite represents a separate and distinctly older 

 intrusion than the syenite-granite, so that the sheetlike arrangement ad- 

 vocated by Bowen is out of the question. But it is by no means necessary, 

 therefore, to assume that both syenite-granite and anorthosite were batho- 

 lithic intrusions. I believe the anorthosite represents a laccolith not 

 much greater across than the present area of outcrop, and that its intru- 

 sion was soon followed by a very irregular intrusion of the great body of 



*s N. L. Bowen : Jour. GeoL, vol. 25, 1917, p. 223. 



« N. L. Bowen : Jour. Geol., vol. 25, 1917, pp. 223-224. 



