TRANSITION ROCKS 449 



How shall be explained the fact that typical Keene gneiss in some 

 places forms belts between syenite or granite and Marcy anorthosite and 

 in other places between syenite or granite and the border (Whiteface) 

 facies of the anorthosite, the Keene gneiss exhibiting almost exactly the 

 same characteristics in each case? If we adopt Bowen^s hypothesis, we 

 are obliged to imagine the development of two sheetlike masses of Keene 

 gneiss, one just above the syenite-granite and the other just below it. Is 

 it not most nnlikely that two masses, formed under such different condi- 

 tions, would be almost exactly the same? This difficulty is obviated by 

 regarding the Keene gneiss as an assimilation product. In my experi- 

 ence, particularly in the Lake Placid quadrangle, Keene gneiss seldom 

 formed except well within the outer margin of the great body of anor- 

 thosite where the temperature was still relatively high. The field rela- 

 tions strongly indicate that assimilation of Whiteface anorthosite took 

 place only where that rock was well within the outer margin — that is to 

 say, where such Whiteface anorthosite was near its change to Marcy 

 anorthosite. 



In the mixed rock areas, where anorthosite has been cut to pieces by 

 intrusions of syenite, the few contacts observed are not very sharp. Ap- 

 parently in these areas either the syenite magma or the anorthosite, or 

 both, were not hot enough, or the syenite was not in sufficient bulk to 

 effect more than slight fusion of the borders of the invaded anorthosite. 



Another important fact is that in the field the Keene gneiss by no 

 means universally forms a narrow zone or belt with syenite-granite di- 

 rectly adjacent on one side and anorthosite on the other. A fine case in 

 point is the eastern part of the Sunrise Notch area (see figure 1), where 

 Keene gneiss for 1% miles lies between granitic syenite on one side and 

 syenite on the other. A different case is the Oak Eidge area northeast of 

 Keene. This is bordered on the south by Whiteface anorthosite and on 

 the north by Grenville, Marcy anorthosite, and syenite. How can areas 

 of this sort possibly be explained by Bowen's hypothesis, which assumes 

 that the anorthosite was never a real magma, but that it was formed by 

 sinking of plagioclase crystals with the development of a transition rock 

 (Keene gneiss) occupying a position distinctly intermediate between the 

 syenite-granite and anorthosite? Is it not much more in harmony with 

 the field relations to conceive that Keene gneiss magma was produced by 

 assimilation at a lower level and then rose to invade previously formed 

 Grenville and anorthosite, or moved upward, flanked on either side by 

 syenite or granite? x\lso, are not elliptical areas like those just east of 

 Upper Jay and li/^ miles west of East Kilns much more satisfactorily 

 accounted for in this manner than by Bowen's hypothesis? Again, do 



