402 W. J. MILLER ADIRONDACK ANORTHOSITE 



the intrusives, the strata are often highly tilted or moderately bent, and 

 in some places locally contorted ; bnt there is no evidence that these rocks 

 have ever been severely folded by orogenic forces. The thickness of the 

 Grenville is certainly at least a few miles. Adams and Barlow report a 

 thickness of abont 18 miles in Ontario. 



My view is that the oldest known intrusive is now represented by the 

 anorthosite, which worked its way upward as a relatively stiff gabbroid 

 magma, probably laccolithic. This rising magma, for most part, lifted or 

 domed the Grenville strata over it, but also, to some extent, engulfed frag- 

 ments of the Grenville. The anorthosite proper is thought to have differ- 

 entiated from the gabbroid magma, as explained beyond in this paper. 



Distinctly later, though apparently not much later, came the intrusion 

 of a tremendous body of magma now represented by syenite-granite series, 

 so common throughout most of the Adirondack region. This vast magma, 

 for most part, slowly and very irregularly worked its way upward, in 

 places actually doming the Grenville, but mostly either breaking up or 

 tilting masses of it, great and small, or breaking through it as great off- 

 shoots from the magma, or more intimately intruding or even injecting it. 



Gushing^ holds "that a complex of Grenville, resting on orthogneiss, 

 existed in the region at the time of the intrusion of the anorthosite-gabbro 

 group ; that much of this orthogneiss still remains in the region, and that 

 the later intrusive broke through this complex in separate masses instead 

 of forming one gTeat body.'' Possibly such an older orthogneiss is present, 

 but I maintain that no real proof for its existence has been thus far pre- 

 sented, and that if it does exist it must be in minor amount. " After many 

 seasons of detailed field-work in the Adirondack area, I have found no 

 positive evidence for the existence of such an older orthogneiss in the 

 areas mapped either by others or by myself.^ In my opinion, the presence 

 or absence of such an older orthogneiss has no important bearing on the 

 problem of the anorthosite, though, as pointed out beyond, if we consider 

 it to be absent we arrive at a somewhat more rational explanation of the 

 origin of the anorthosite. I have never maintained, as Gushing seems to 

 imply, that the anorthosite and the syenite-granite series are differentiates 

 of a single great intrusive body. With Gushing, I have always believed 

 the anorthosite to be a distinctly earlier intrusive, but I do not believe it 

 has ever been proved that what we regard as the great syenite-granite 

 series v^as intruded as distinctly separate masses, as Gushing implies. 

 Here, again, I am open to conviction; but in all my experience I have 



5 H, p. Gushing : Tour. Geol., vol. 25, 1917, p. 504. 



8 The Thousand Islands district, really beyond the Adirondack Mountain area proper, 

 is not considered in this paper. 



