424 W. J. MILLER ADIRONDACK ANORTHOSITE 



quadrangle I found none at all. Gushing shows none on his Long Lake 

 map. 



On his Elizabethtown-Port Henry map, Kemp represents an area of 

 Grenville, three-quarters of a mile long, completely surrounded by Marcy 

 anorthosite. Several other small areas are shown to lie between the basic 

 border and the Marcy anorthosite. Kemp has also described some inclu- 

 sions of Grenville (too small to be mapped) in anorthosite, presumably 

 the Marcy type, as occurring in the Elizabethtown quadrangle. He has 

 also observed many small inclusions of Grenville gneiss in massive anor- 

 thosite, presumably the Marcy type, in the vicinity of Keene Valley, in 

 the Mount Marcy quadrangle.-^ 



The few known inclusions of Grenville in Marcy anorthosite, above de- 

 scribed, all lie close to its border facies. It is, then, a striking fact that 

 the Whiteface (border) anorthosite contains many inclusions and consid- 

 erable areas of Grenville, while the Marcy anorthosite contains very few, 

 and these near the border facies. The explanation is simple. Only the 

 border phase of the great body of intruding magma actually came in con- 

 tact with or broke into the country rock (Grenville) to any notable ex- 

 tent. Masses of Grenville, large and small, were torn from both the sides 

 and roof of the magmatic chamber. Many of these fragments sank into 

 or were enveloped by the border facies, which I regard as having devel- 

 oped both as an outer and an upper limit, while but few of them ever 

 sank or were forced far enough into or through the border phase of the 

 intruding magma to reach even the outer portion of what developed into 

 typical Marcy anorthosite. Now, the few known inclusions of Grenville 

 in the Marcy anorthosite are close to the border phase and hence close to 

 its inner (or lower) limit. Theoretically, at least, it is not impossible to 

 imagine some Grenville fragments farther within the Marcy anorthosite, 

 where the border facies was relatively thin, or due to shifting of some of 

 the typical anorthosite magma closer to the upper or outer limit of the 

 magma chamber by magmatic currents during a later stage of consolida- 

 tion. Possibly the small inclusions of Grenville in anorthosite in the 

 vicinity of Keene Valley above referred to are to be explained in this way. 



It is therefore easy to understand why no inclusions of Grenville are 

 definitely known well within the great body of Marcy anorthosite, and 

 this in spite of much detailed work, because not only the upper or border 

 phase of the anorthosite, which probably contained many inclusions, but 

 also much of the upper portion of the Marcy anorthosite have been re- 

 moved by deep and widespread erosion over the region. 



29 J. F. Kemp : Bull. Geol. Soc. Am., vol. 25, 1014, p. 47. 



