412 ^y. j. miller — Adirondack axorthosite 



not uncommonly exhibits a magmatic flow-strncture foliation, the labra- 

 dorite cr3'stals having been strung out into crude parallelism in a 3'et 

 molten portion of the rock, ^ot onl}^ was this interstitial liquid in suffi- 

 cient quantity to permit the development of distinct magmatic flowage,' 

 but it was essentially molten plagioclase. It could have been nothing 

 else. Hence we here have evidence directly opposed to Bowen's statement 

 that the anorthosite was never at a temperature sufficiently high to melt 

 jDlagioclase. The temperature was certainly high enough to keep some 

 of the plagioclase in a molten condition, and such a temperature could not 

 have been much lower than would have been necessary to keep all the 

 plagioclase molten. This flow-structure foliation may well have devel- 

 oped during a late stage of magma consolidation, and it by no means 

 proves that the whole mass of plagioclase was never molten as such dur- 

 ing an earlier stage. At any rate, I believe that, in view of the composi- 

 tion, variability, and foliation of the anorthosite, Bowen does not allow 

 for the presence of enough fluid in the cooling intrusive. How can the 

 many notable variations of the anorthosite, often within very short dis- 

 tances, and the common occurrence of foliation, with exceedingly variable 

 strikes and dips, be otherwise accounted for ? It is utterly impossible for 

 me to imagine how such features could have resulted from the sinking of 

 plagioclase crystals, giving rise to a great mass of anorthosite, never, to 

 at least a very considerable degree, molten as such. 



But Bowen-^ says : "It seems reasonably possible both to obtain and to 

 maintain almost any degree of heterogeneity as a result of the accumula- 

 tion of crystals." He suggests that those portions relatively richer in 

 plagioclase or in femic minerals accumulated in different portions of the 

 (gabbroid) magma reservoir, and that they were, as partly crystalline 

 masses, thrust into the positions they now occupy. Does Bowen mean 

 that portions relatively rich in plagioclase, and others rich in femic con- 

 stituents, tended to concentrate locally in many places throughout the 

 cooling magma, from which positions they were, in many cases, shifted 

 by magmatic flow ? If so, I am inclined to look with favor on this exjjla- 

 nation of the alternations of more or less clearly defined bands or zones 

 of more gabbroid anorthosite with others of practically pure plagioclase. 

 There are, as already pointed out, many portions, unusually rich in femic 

 minerals, of irregular size, shape, and distribution throughout the anor- 

 thosite. These are not sharply separated from adjacent anorthosite much 

 poorer in, or even practically free from, femic minerals. Some such more 

 femic portions are foliated and others are not. They are almost certainly 

 results of local differentiation wherebv the femic constituents were here 



X. L. Bowen : Jour. Geol., vol. 25, 1917, p 237. 



