RELATION OF ANORTHOSITE TO GRENVILLE SERIES 425 



The inclusions of Grenville are significant in yet another way. Since 

 many of them occur well within or toward the inner margin of the White- 

 face anorthosite, or even just beyond it in the outer portion of the body 

 of Marcy anorthosite, it is evident that the Grenville inclusions, both 

 small fragments and larger masses, must either have been torn off by and 

 enveloped in a very active anorthosite intrusive magma, or they must 

 have settled through at least a few thousand feet of anorthosite which 

 was in a pretty highly fluid state. It might be argued that these inclu- 

 sions were enveloped by or sank well into an original gabbroid magma, 

 from which, by settling of the femic constituents due to gravity, the 

 anorthosite, never molten as such, developed. But, on this view, why 

 should the femic constituents on crystallizing have settled, when the in- 

 clusions, many of them made up of heavy femic minerals, remained sus- 

 pended? Again, it should be remembered that many of the Grenville 

 fragments show an arrangement, even in almost pure plagioclase anor- 

 thosite, parallel to what I confidently believe to be a magmatic flow- 

 structure foliation. This argues for a former true magmatic condition 

 of this sort of anorthosite with a very considerable amount of liquid — 

 that is to say, enough to have permitted distinct magmatic currents. 

 Now, if this j^ractically pure plagioclase anorthosite once possessed such 

 a high degree of fluidity, is it out of the question to argue that the nota- 

 bly more impure general body of Marcy anorthosite may once have been 

 in a true magmatic condition? 



The anorthosite of the Eainy Lake district of Ontario furnishes impor- 

 tant corroborative evidence. That this rock was once in magmatic form 

 and very actively intrusive is Coleman's*^*^ view, stated as follows : 



"Frequently portions of chloritic or sericitic schist have been inclosed by the 

 anorthosite (consisting of fully nine-tenths plagioclase), showing its post- 

 Keewatin age, and occasionally a green massive rock, apparently weathered 

 diabase, is seen, probably portions of massive Keewatin rocks swept off by the 

 molten anorthosite." 



DIKES OF ANORTHOSITE IN GRENVILLE 



Bowen says,^^ regarding the Adirondack anorthosite, that none of the 

 "investigators who have studied the area have found a single dike of 

 nearly pure plagioclase in the surrounding rocks," and that "it seems to 

 be a reasonable conclusion that this material was incapable of being in- 

 jected into the older rocks." He admits that anorthosite-gabbro invades 

 the Grenville in places. These statements call for consideration. 



A. p. Coleman : Jour. Geol., vol. 4, 189G, p. 911. 

 N. L. Bowen : Jour. Geol., vol. 25, 1917, p. 220. 



