CHILLED GABBROID BORDER FACES 415 



thosite, with perfect gradations from one into the other, and with no evi- 

 dence whatever that syenite or granite was ever developed as a rock inter- 

 mediate between the border phase and the true anorthosite. Though it 

 has been notably cut into and partly assimilated by the syenite-granite 

 body, a glance at the geologic map shows beyond question that this White- 

 face anorthosite was formerly a continuous border phase of the body of 

 Marcy anorthosite which occupies the whole northeastern one-third of the 

 quadrangle. Three masses of the Whiteface anorthosite, from 1 to 2 

 miles wide and from 2 to 4 miles long, still lie against the Marcy anor- 

 thosite in their original undisturbed positions. There is strong evidence 

 that this border phase v^as formerly at least 7 or 8 miles wide, because 

 within that distance out from the typical Marcy anorthosite many smaller, 

 widely scattered masses of rather gabbroid Whiteface anorthosite occur 

 as mappable inclusions in the syenite-granite series all the way across the 

 quadrangle. In other words, only remnants of the original border rock 

 now remain. 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 comparable in origin and position to Cushing's 

 Long Lake border rock, though of lighter color and usually not so thor- 

 oughly gabbroid. The Schroon Lake quadrangle Whiteface anorthosite 

 certainly averages more gabbroid than the Marcy anorthosite. It com- 

 monly carries 10 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. 



My detailed survey of the Lake Placid quadrangle shows an unusually 

 extensive development of Whiteface anorthosite, there being at least 40, 

 and possibly 50, square miles, considerable areas being more or less effec- 

 tually masked by glacial drift. One area alone is 10 miles long, with a 

 maximum width of nearly seven miles. Among the other areas three are 

 each several miles long. The Whiteface anorthosite of this quadrangle 

 usually contains 10 to 12 per cent dark minerals, and it is therefore only 

 a little more gabbroid than the Marcy anorthosite. But it varies greatly, 

 some portions carrying almost no femic minerals and others 20 per cent or 

 more. It has a very irregular distribution, not forming a definite fringe 

 or outer margin of the Marcy anorthosite, as in most other districts. Fur- 

 ther, it has been very extensively cut into by intrusions of the syenite- 

 granite series, as explained in detail below. In no case, liowevcr, was any 

 syenite found to be transitional between the Whiteface and Marcy anor- 

 thosites. Although the Whiteface anorthosite of the Lake Placid quad- 

 rangle is distinctly less gabbroid than most of the known border facies 

 elsewhere, and its distribution is so irregular, it is, nevertheless, quite 



XXXII— Bull. Geol. Soc. Am.. Vol. 29, 1917 



