26 NEW YORK STATE MUSEUM 



above pointed out, the border fades (Whiteface type) there 

 formerly extended fully 7 or 8 miles out beyond the present margin 

 of the Marcy anorthosite as indicated by numerous inclusions in 

 the syenite-granite series. In this connection, a very interesting 

 inclusion of fragments of very typical Marcy anorthosite in the 

 granite of Wilson mountain, over 6 miles out from the present 

 border of that type of anorthosite, may be reasonably interpreted 

 as Marcy anorthosite caught up in the granite magma at a lower 

 level (below the Whiteface anorthosite cover) and carried upward 

 to the present position (see figure i). In any case it is certain that 

 Marcy anorthosite existed that far out. 



Within the Schroon Lake quadrangle no Whiteface anorthosite 

 was found within the large area of Marcy anorthosite, it apparently 

 all having been removed by erosion. Unless definite areas of the 

 basic chilled border facies are found far within the great anortho- 

 site area, positive proof that such a border once existed as a cover 

 over the whole will be wanting. But such a cover, if once uni- 

 versally present,- would show few, if any, remains far within the 

 anorthosite area because of the widespread and deep erosion to 

 which the region has been subjected. 



In short, the evidence from the outer portions of the great 

 Adirondack anorthosite body strongly supports the view that a 

 chilled gabbroid border facies should be regarded as having 

 formerly existed as a cover resting upon the whole mass of Marcy 

 anorthosite. The evidence from the interior is negative, but nothing 

 in the field is opposed to the conception of a former universal 

 cover. But this does not preclude Cushing's conception of an 

 outer chilled border of the anorthosite, provided we regard the 

 anorthosite as a great laccolithic intrusive body (see figure 2) 

 over all of which a border facies developed as an upper limit, 

 and at the margins of which a border facies developed at the same 

 time as an outer limit. The writer therefore agrees 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. 



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) remained practically suspended. At a later stage, 

 when the liquid became light enough, plagioclase crystals (then in 



