426 W. J. MILLER ADIRONDACK ANORTHOSITE 



I admit that there is no definitely known example of a true dike of 

 typical Marcy anorthosite in the older rocks. Now, any such dikes must 

 be looked for in the Grenville rocks because these were the only ones 

 present at the time of the intrusion. But, as already pointed out, Marcy 

 anorthosite and Grenville are known to come together in very few places, 

 and these are where mere inclusions of Grenville (some of mappable size) 

 occur. It is therefore not at all conclusive to say that because dikes of 

 Marcy anorthosite are unknown this anorthosite was never molten as such. 

 It should also be remembered that I maintain that the Marcy anorthosite 

 was once either wholly, or at least largely, molten, but in a rather viscous 

 condition, and hence not so favorable for intrusion as small dikes into the 

 few small bodies of Grenville which it is known to have enveloped. 



With the border facias of the anorthosite the case is notably different, 

 because it was the outer and upper portion of the great intrusive body 

 which came into direct contact with the Grenville country rock, and hence 

 was favorably situated for cutting into and enveloping masses of Gren- 

 ville. As above shown, particularly in the areas of Grenville and White- 

 face anorthosite mixed rocks of the Lake Placid quadrangle, dikes and 

 bands of Whiteface anorthosite in Grenville commonly occur and many 

 of them are sharply defined. It should be repeated with emphasis, that 

 some of these dikes and bands are certainly fully as free from femic min- 

 erals as much of the very t3^pical Marcy anorthosite. This field evidence, 

 therefore, does not support Bowen's statement regarding absence of dikes 

 of nearly pure plagioclase. 



In the other quadrangles which have been mapped more or less in de- 

 tail actually observed contacts between Grenville and anorthosite are very 

 rare and the existence of anorthosite dikes is neither proved nor dis- 

 proved. In Franklin County, Cushing^^ has found that "the few contacts 

 exposed show the anorthosite cutting them (Grenville and other older 

 gneisses) and sending tongues into them," but the exact kind of anor- 

 thosite is not stated. 



Eelation of the Syenite-granite Series to the Anorthosite 



syenite-granite series distinctly younger than the anorthosite 



According to Bowen, the syenite-granite series and the anorthosite are 

 not distinctly separate intrusives, but both formed as differentiates from 

 a single great body of intruded gabbroid magma. He^^ says : "I think it 

 must be admitted that in much of the quadrangle work the syenite is 



32 H. p. Gushing : N. Y. State Mus. Bull. 95, 1905, p. 307. 



33 N. L. Bowen: Jour. GeoL, vol. 25, 1917, p. 511. 



