GEOLOGY OF THE LAKE PLACID QUADRANGLE 3 1 



extended at least a few miles farther out than the present boundary. 

 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 anortho- 

 site. 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 Whiteface 

 anorthosite merely an outer border facies. But if we do so regard 

 this border facies, we are forced to conclude that it is exceedingly 

 thick — that is to say, fully 10 or 12 miles — the width of the area 

 containing Whiteface anorthosite representing practically the thick- 

 ness of the border phase. This is scarcely possible. If, however, 

 we consider the Whiteface anorthosite of the quadrangle to mark 

 an upper limit of the great anorthosite body, but now partially 

 removed by erosion and partly cut into by the syenite-granite series, 

 not nearly so great thickness need be assumed. On this view a 

 vertical thickness of fully 3000 feet is actually exposed in Mt 

 Whiteface, and how much more should be added to make up for 

 the upper portion removed by erosion is of course unknown. Prob- 

 ably little or none is to be added to the bottom, because Marcy 

 anorthosite outcrops near the base of the mountain. 



The anorthosite younger than the Grenville, There is two- 

 fold evidence that the anorthosite is younger than the Grenville 

 series, namely, distinct inclusions of Grenville rocks in the anortho- 

 site, and the more or less intimate penetration of the Grenville 

 by the anorthosite, particularly by the Whiteface anorthosite. 



Inclusions of Grenville in the Marcy anorthosite seem to be 

 uncommon, having been noted in only one locality, namely, one- 

 fourth to one-half of a mile north of Red rock in the vicinity 

 of Keene, where a number of sharply defined, small inclusions of 

 Grenville quartz-pyroxene gneiss and quartzite, and one 2-foot 

 inclusion of limestone, are plainly imbedded in the typical Marcy 

 anorthosite. The writer recalls having seen a lO-foot inclusion 

 of limestone in anorthosite on the shore of Long lake in the Blue 

 Mountain quadrangle (see Museum Bulletin 192). 



In the Whiteface anorthosite, however, inclusions of Grenville 

 occur in many places. Only a few of the better, more accessible 

 localities will be cited. A 20-foot inclusion of nearly white Gren- 

 ville quartz-feldspar-garnet gneiss clearly shows in a big ledge just 

 east of Keene. Others, including quartzite and green pyroxene 

 gneiss, occur for a mile eastward on the way up the hill east of 

 Keene and along its summit. 



