CHILLED GABBROID BORDER FACES 419 



is negative, but nothing in the field is opposed to this view. I do not 

 believe this view precludes Cushing's conception of an outer chilled border 

 of the anorthosite, provided we regard the anorthosite as a great lacco- 

 lithic intrusive body (see figure 3), all over which a border facies de- 

 veloped as an upper limit, and at the margins of which a border facies 

 developed at the same time as the outer limit. I therefore agree with 

 Gushing that the area of anorthosite shown on the State geologic map 

 shows practically "the original size of the mass at the depth represented 

 by the present erosion surface/' and that the anorthosite can not extend 

 out to, or even close to, the margins of the whole Adirondack region. 



Although Bowen, in his second paper, admits the probable existence of 

 a chilled gabbroid border facies, his conception is essentially different 

 from mine in two important respects : First, he believes the border facies 

 represents wholly an upper limit of a laccolithic intrusive body as large 

 as the whole Adirondack region, and, second, he maintains that the sye- 

 nite-granite series developed between the border facies and the true anor- 

 thosite still lower down, and that much of this syenite-granite series must 

 still exist in that position, though much of it was later by reintrusion 

 forced through the gabbroid cover. In the light of the field facts above 

 presented, I am decidedly opposed to these views of Bowen. 



SIGNIFICANCE OF THE CHILLED BORDER FACIES 



According to Bowen, the femic constituents of a great gabbroid magma 

 as wide as the Adirondack region first separated (or sank) by gravity, 

 while the plagioclase crystals (then in the form of basic bytownite) re- 

 mained practically suspended. Then, at a later stage, when the liquid 

 became light enough, plagioclase crystals (then in the form of labrador- 

 ite) accumulated by sinking, thus giving rise to the mass of the anortho- 

 site, leaving the overlying liquid of such composition as to yield syenite 

 or granite. In his first paper Bowen does not consider the development 

 of a chilled border of the Adirondack anorthosite. In his second paper, 

 by way of reply to Gushing, he modifies his idea of the stratiform arrange- 

 ment of the igneous complex by considering the development of a "gab- 

 broid chilled upper portion of a laccolithic mass extending far beyond the 

 limits of the present exposure." Directly under the chilled border, ac- 

 cording to Bowen, the great body of syenite-granite developed ; still lower 

 down the typical anorthosite formed, and at the bottom pyroxenite and 

 gabbro. 



Now, Professor Gushing and I both have demonstrated that the great 

 body of anorthosite has a gabbroid chilled border which can not possibly 

 extend far out beyond the present exposure of the anorthosite, and with 



