GEOLOGY OF THE SCHROON LAKE QUADRANGLE 2$ 



the geologic map shows beyond question that this Whiteface anor- 

 thosite was formerly a continuous border phase of the Marcy anor- 

 thosite which latter occupies the whole northeastern one-third of 

 the quadrangle. Three large bodies of the Whiteface anorthosite 

 still lie against the Marcy anorthosite in their original positions. 

 There is strong evidence that this border phase was formerly at 

 least 7 or 8 miles wide because, within that distance out from 

 the Marcy anorthosite, many smaller widely scattered masses of 

 the Whiteface rock occur as inclusions in the syenite-granite series 

 all the way across the quadrangle. In other words, only remnants 

 of the original border rock now occur. Further, since this border 

 rock is notably finer grained than the Marcy anorthosite, it is very 

 reasonable to interpret it as a chilled gabbroid border phase com- 

 parable in position and origin to Cushing's Long Lake border 

 phase of the anorthosite, though of lighter color and usually not 

 so gabbroid. The Schroon Lake quadrangle Whiteface anorthosite 

 commonly carries lO to 20 per cent dark minerals, but it is very 

 variable, some phases containing only 5 per cent or even less, and 

 some more than 20 per cent. 



There is strong evidence that the chilled gabbroid border phase 

 developed not only as an outer limit but. also as an upper limit 

 which formerly existed as a cover resting directly upon the whole 

 great mass of anorthosite. Thus, as already pointed out in the 

 writer's Lake Placid report, the Whiteface anorthosite of that area 

 does not exist merely as a definite fringe around the outer margin 

 of the Marcy anorthosite. Whiteface anorthosite there occurs 

 fully 14 or 15 miles within the present border of the anorthosite 

 area, and inclusions in the syenite-granite series outside the gen- 

 eral anorthosite area show that the Whiteface anorthosite formerly 

 extended at least a few miles farther out than the present bound- 

 ary. One area of Marcy anorthosite, 12 miles long within the Lake 

 Placid quadrangle and extending an unknown distance into the 

 Ausable quadrangle, is flanked on either side by Whiteface anor- 

 thosite. It is hard to resist the suggestion that the Whiteface rock 

 formerly covered this whole mass of Marcy anorthosite. There 

 is thus a distinct difficulty in the way of considering this White- 

 face anorthosite as merely an outer border facies. If we do regard 

 it as merely an outer facies, we are forced to conclude that it is 

 exceedingly thick, that is to say fully 10 or 15 miles, the width of 

 the area containing Whiteface anorthosite representing practically 

 the thickness of the border facies. This is scarcely conceivable. 



The Schroon Lake quadrangle yields similar evidence since, a§ 



