THE ADIRONDACK MOUNTAINS 37 



have apparently been left intact and undisturbed. These igneous 

 rocks are all of the plutonic type ; that is, they were never forced up 

 to the earth's surface but solidified at considerable depths below the 

 surface. We see them exposed today only because a tremendous 

 amount of overlying material has been removed by erosion. These 

 igneous rocks are generally easily distinguished from the old sedi- 

 ments of Grenville age because of their more general homogeneity 

 in large masses and their lack of sharply defined bands of varying 

 composition. The fact that the minerals have always crystallized 

 to form medium to coarse grained rocks shows that the rocks solidi- 

 fied under deep-seated conditions, since it is well known that surface 

 flows (lavas) are much finer grained and often with more or less 

 of the rock not crystallized at all. Slow cooling under great pressure 

 favors more complete crystallization and the growth of larger 

 crystals. 



The first of the two well-known great intrusions of molten rock 

 in the Adirondacks is represented by the present large area (1200 

 square miles) of so-called anorthosite^ mostly in Essex and Franklin 

 counties. The typical rock is very coarse grained, of bluish gray 

 color and consists principally of a feldspar (labradorite). This 

 feldspar is in crystals commonly from one-half of an inch to nearly 

 2 inches long with shiny cleavage faces when freshly broken, and 

 they often exhibit a pretty play of colors. Fine parallel fines, as 

 though ruled on with a delicate instrument, also very commonly 

 show on the cleavage faces. The typical anorthosite is usually 

 quite homogeneous in large bodies as, for instance, in the Sentinel 

 range a few miles east of Lake Placid, and in the mountains just 

 west of the same lake. An interesting and important variation of 

 the typical rock is known as the Whiteface type of anorthosite, so 

 named from Mt Whiteface, the summit of which is made up of this 

 rock. This is usually white or nearly so, the feldspar crystals 

 having been much broken (granulated) under pressure. Another 

 variation is a darker rock due to a considerable percentage of black 

 minerals, especially pyroxene, amphibole and iron ore. This darker 

 rock is often rather streaked in appearance, the minerals having 

 been drawn out into crude parallelism due, no doubt, to move- 

 ments or currents in the molten masses just before they congealed. 

 This variety of anorthosite is only locally developed, especially 

 around the borders of large bodies of the typical anorthosite. Many 

 of the highest mountains of the east-central Adirondacks, such as 

 Marcv. Mclntyre and Whiteface, consist wholly or largely of anor- 

 thosite. The anorthosite intrusion differed from the later intrusions 



