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GEOLOGY OF THE LAKE PLACID QUADRANGLE 2^ 



must be present up to 5 per cent or more as untwinned feldspar. 

 Gushing, however, says : " The potash is in the labradorite (or 

 other plagioclase), replacing a certain amount of soda" and "that 

 analyses of this feldspar always show it." If orthoclase is present 

 in the typical anorthosites, the writer has been unable to demon- 

 strate it in the thin sections examined. Usually the plagioclase is 

 beautifully and completely twinned, but many of them are much 

 less so, some of the slices showing only a little very local, very faint 

 twinning. Still other feldspar slices, which show no twinning but 

 which have apparently similar index of refraction and double 

 refraction, are quite certainly untwinned plagioclase. It seems 

 most probable, therefore, that much, if not all, of the potash is in 

 the plagioclase, with possibly a little in the dark minerals pyroxene 

 and hornblende. Certain border phases of the anorthosite (below 

 described) do contain orthoclase and microperthite, but these are 

 believed to be due to mixing with the later syenite-granite magma. 



Graphite in the anorthosite. Professor Kemp has in a letter 

 furnished the following information regarding some interesting 

 occurrences of graphite in the anorthosite. " Below the dam across 

 the Saranac river at Franklin Falls, in the ledge on which it rests, 

 there is a streak of graphite traceable for some yards. In several 

 places on Knapp hill, 2 miles to the south, I found the graphite 

 again in the Whiteface type. I have also found it in the Marcy type 

 of anorthosite where exposed near the Grenville at the Red Rocks 

 (near Keene). Probably the hydrocarbons which yielded the 

 graphite were stewed out of the neighboring or included Grenville. 

 This mineral is unusual in intrusive rocks." 



The anorthosite an intrusive body. Until very recently, the 

 Adirondack anorthosite has been regarded by the several workers 

 in the region as an intrusive in the ordinary sense of that term. 

 N. L. Bowen has, however, seriously questioned whether the anor- 

 thosite ever had been a truly molten mass. His reasons for think- 

 ing that it was never hot enough to have been really molten are 

 based upon certain chemical considerations. His study of the 

 literature led him to believe that his conception is not opposed by 

 field facts. He stresses the simple mineral composition of the anor- 

 thosite and says in part: " Normally rocks are made up of several 

 minerals, and when considering their magmas, we have to regard 

 the various minerals as existing therein in mutual solution. . . . 

 What, then, of the solution theory as applied to the anorthosites 

 which typically consist almost exclusively of the single mineral 

 plagioclase? Were they ever hot enough to be molten per sef 



