436 ^V. J. MILLER ADIRONDACK AXORTHOSITE 



known because of lack of detailed field-work. In any case, if there is no 

 direct surface connection, the space between must be narrow. 



In the second place, the syenite of these bodies grades into granitic 

 syenite and even into granite, and is in every w^ay exactly like the usual 

 syenite throughout the Adirondack region. 



In the third place, I have j^roved that the Lake Placid syenite-granite 

 mass is intrusive into both the Whitef ace and Marcy types of anorthosite, 

 as shown by the dikes, inclusions of anorthosite in the syenite, and the 

 irruptive contacts (see above). There is considerable evidence of some- 

 what finer grained syenite and granite due to border chilling near the 

 contacts with the older rocks. The distribution of this syenite-granite 

 mass with respect to the surrounding rocks also strongly indicates its 

 intrusive character. It is also very distinctly intrusive into Grenville, as 

 proved both by inclusions and irruptive contacts. In short, the evidence 

 that this body of syenite-granite is really intrusive into and distinctly 

 later than the anorthosite is fully as strong as Cushing's evidence that 

 the syenite of the Long Lake quadrangle is distinctly younger than the 

 anorthosite there. 



In the fourth place, as already shown, the chilled border facies of anor- 

 thosite certainly developed as a marginal mass lying directly against and 

 grading into the typical ^larcy anorthosite. This is clearly true of the 

 border facies (Whitef ace anorthosite) in the Lake Placid quadrangle. 

 There is no positive evidence that S3^enite ever developed as a rock inter- 

 mediate in position and character between the border facies and the Marcy 

 anorthosite of the quadrangle. A seeming exception is the broad belt 

 between Marcy and Whitef ace anorthosite north of Lake Placid; but the 

 facts that this is only a part of the clearly intrusive body both to the 

 north and the south, and that transition rocks are absent, make its intru- 

 sive character certain. The narrow belt of transition rock between the 

 syenite and anorthosite on the Sentinel Eange is much more plausibly 

 explained as due to assimilative attack on the anorthosite by distinctly 

 later syenite magma (see discussion of "Keene gneiss'^ below) rather than 

 differentiation in situ. 



In the fifth place, the foliation of this Lake Placid syenite-granite mass 

 is usually distinct and, as I have elsewhere shown,**^ this is a magmatic 

 flow-structure foliation. The dips are usually high, indicating a notable 

 upward movement of the magma, such as would scarcely be expected to 

 have taken place with a magma developed by differentiation in situ be- 

 tween the border facies of the anorthosite and the Marcv anorthosite. 



" W. J. Miller : Jour. Geol., vol. 24, 1916, pp. 000-612. 



