GEOLOGY OF THE LAKE PLACID QUADRANGLE 1 7 



It is prominently developed in the Lake Placid quadrangle. The 

 typical rock, known as the Marcy anortliosite, is very coarse 

 grained, bluish gray in color, and consists principally of basic 

 plagioclase feldspar. A widely developed facies, known as the 

 Whiteface anorthosite, is usually medium grained and character- 

 ized by a preponderance of milky white to light, greenish gray, 

 basic plagioclase feldspar with small amounts of dark minerals. 

 Hoth of the types locally contain considerable quantities of dark 

 minerals. For most part the molten anorthosite appears to have 

 pushed aside or displaced the Grenville rocks, though in many cases 

 masses of Grenville, from small fragments to large bodies, were 

 enveloped by the anorthosite, and in still other cases there appears 

 to have been intimate injection of the Grenville by the molten 

 anorthosite. 



Marcy type of anorthosite. Distribution. This type of the 

 anorthosite is named from Mt Marcy where the rock is so well 

 developed. When the whole great body of Adirondack anorthosite 

 is considered, this Marcy type is the most commonly and typically 

 developed. Within the Lake Placid quadrangle, however, a special 

 phase, known as the Whiteface anorthosite (see below) is actually 

 somewhat more abundant in the known areas of outcrop. 



The areas colored to show the extent of the Marcy anorthosite 

 represent about 35 square miles wholly confined to the southern 

 two-thirds of the quadrangle. There are ten areas in all. By far 

 the largest exposed body of Marcy anorthosite in the quadrangle 

 extends from the northern portion of the Sentinel range north- 

 eastward to the village of Haselton. Next to the largest body 

 extends northwestward from Lake Placid for several miles. Two 

 small bodies occur on the large islands in Lake Placid. Other 

 small masses are located as follows : on Marble mountain ; west of 

 Wilmington village; and four small ones in the vicinity of Keene 

 village. In addition to these definitely known areas, some Marcy 

 anorthosite quite certainly lies concealed under Pleistocene deposits 

 in the areas mapped as occupied by heavy glacial and postglacial 

 deposits. 



Megascopic features. The most common phase of Marcy anor- 

 thosite is a very coarse-grained, light to dark bluish gray rock con- 

 sisting very largely of basic plagioclase feldspar, mainly labradorite. 

 The dark bluish gray labradorite crystals usually vary in length 

 from one-fourth of an inch to several inches, with crystals an inch 

 long very common. Occasionally the labradorites exhibit the 

 beautiful play of colors (chiefly green and blue) so characteristic 



