

GEOLOGY OF THE SCHROON LAKE QUADRANGLE 23 



In the whole area from Blue Ridge mountain eastward to the 

 map limit, there are many excellent exposures of remarkably uni- 

 form, very typical Marcy anorthosite, there being few gabbroid 

 or foliated variations in this large area. 



Where the road crosses the Branch brook a ledge of typical 

 Marcy anorthosite contains an irregular gabbroid mass about 2 

 feet wide without sharp boundaries against the inclosing rock. 



Variations similar to those just described were observed in many 

 other places, but enough have been described to illustrate the nature 

 of the variability of the anorthosite. 



Significance of- the composition and variations of the Marcy anor- 

 thosite. According to Bowen, " anorthosites are made up almost 

 exclusively of the single mineral plagioclase " and therefore " the 

 conception of the mutual solution on minerals in the magma and 

 the lowering of temperatures consequent thereon is no longer 

 applicable." But, in view of the facts above presented which show 

 that the anorthosite averages fully 10 per cent femic minerals 

 visible to the naked eye ; that the labradorites carry myriads of 

 tiny inclusions of a dark mineral ; and that the anorthosite contains 

 a notable percentage of potash, is the mutual solution theory neces- 

 sarily precluded? Have we any proof that a rock with such a 

 quantity and variety of constituents other than plagioclase could 

 not have been, largely at least, molten as such? Is it safe to argue 

 from experiments on small amounts of rather pure melts under 

 ordinary laboratory conditions that a rock like the Adirondack 

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

 that " a rock containing 10 per cent diopside (and 90 per cent plagio- 

 clase) could have had a maximum of 35 per cent liquid " in an arti- 

 ficial 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 orthoclase in the liquid." But the Adirondack 

 anorthosite would have formed a melt of notably more compli- 

 cated composition than an artificial melt with 10 per cent diopside, 

 and this 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? Furthermore, allowance should be 

 made for various agencies well within the earth, particularly dis- 

 solved vapors the escape of which pressure tends to prevent, and 

 which tend to increase fluidity. 



Since the foliation of the anorthosite is essentially a magmatic 

 flow-structure, it shows that, at the very least, large portions of the 



