410 W. J. MILLER ADIRONDACK AXORTHOSITE 



As a result of his work on the western side of the anorthosite body, 

 Cushing"^^ states that changes from typical anorthosite to anorthosite- 

 gabbro, or even gabbro, "take place here and there within the general 

 anorthosite mass/' 



Another variation from the anorthosite as a pure plagioclase rock con- 

 sists in the dustlike (schillerization) inclusions of a dark mineral, prob- 

 ably ilmenite, in the labradorite. These are so numerous as to cause most 

 of the labradorites to have a dark color. Thus even the plagioclase itself 

 is not pure soda-lime feldspar. Adams^^ says, regarding the Morin anor- 

 thosite, that the "labradorite is filled with an infinite number of minute 

 schillerization inclusions, which give to it a deep violet or nearly black 

 color, so that the massive anorthosite is always very dark." Lawson de- 

 scribes similar numerous minute inclusions as occurring in many of the 

 labradorites of the anorthosite of the Minnesota coast of Lake Superior. 



Finally, in this connection, I would call attention to the presence of 

 the very appreciable amount of potash in the typical anorthosite (see 

 analysis above). Whether this potash exists in regular potash feldspar 

 form or forms part of the labradorite proper, it is an additional proof 

 that the anorthosite is not a practically pure mass of lime-soda feldspar. 



In view of the facts, therefore, that the anorthosite averages fully 10 

 per cent of femic minerals visible to the naked eye; that its labradorites 

 carry myriads of tiny inclusions of a dark mineral, and that the anor- 

 thosite contains a notable percentage of potash, is the mutual solution 

 theory necessarily precluded, as argued by Bowen? Have we any proof 

 that a rock with such a quantity and variety of constituents other than 

 lime-soda feldspar could not have been, largely at least, molten as such? 

 Is it safe to argue from experiments on very small amounts of rather pure 

 melts under ordinary laboratory conditions that a rock like the Adiron- 

 dack anorthosite could not have existed as a true magma? Bowen says 

 that "a rock containing 10 per cent diopside (and 90 per cent plagioclase) 

 could have had a maximum of 35 per cent liquid" in an artificial melt, 

 and that in a natural melt "the probability is that the amount of liquid 

 would be relatively somewhat larger on account of the presence of ortho- 

 clase in the liquid." But the Adirondack anorthosite would have formed 

 a melt of notably more complicated composition than the artificial melt 

 with 10 per cent diopside and under deep-seated geologic conditions. Is 

 it safe to say, therefore, that such a melt may not have been a true magma 

 with a high percentage of liquid ? 



Bowen argues that anorthosite, being made up almost exclusively of the 



18 H. p. Gushing : N. Y. State Mus. Bull. 115, 1907, p. 473. 



" F. D. Adams : Geol, Sur. Can., Guide Book No. 3, 1913, p. 9. 



