TRANSITION ROCKS 447 



to include fragments of the anorthosite and in other places to yield irrup- 

 tive contacts against the anorthosite at higher levels. 



SIGNIFICANCE OF THE DI8TRIBUT0N OF THE KEENE GNEISS 



That the Keene gneiss is actually an assimilation rock, the product of 

 fusion and digestion of anorthosite by syenite or granite magma, is re- 

 garded proved by the evidence above presented. It can not be a direct 

 differentiate of either the syenite-granite series or the anorthosite, be- 

 cause it never occurs except on the border or close to the contact between 

 the syenite or granite and the anorthosite. But such rock is not univer- 

 sally present. For instance, the long boundaries between the Whiteface 

 anorthosite and granite of Mount Whiteface and between the Whiteface 

 anorthosite and syenite from the southern side of Mount Whiteface to 

 west of Knapp Hill were crossed by me at a good many places without 

 noting any rock like the Keene gneiss. Some other places also have no 

 Keene gneiss as a border rock. It is probable that some masses of Keene 

 gneiss may have been overlooked in the rough, densely wooded country, 

 and that some may lie under cover of Pleistocene deposits; but, in view 

 of the detailed surveys of the Lake Placid and Schroon Lake quadrangles, 

 it is certain that any such masses must be relatively small in those dis- 

 tricts. 



By v^ay of contrast with the conspicuous development of Keene gneiss, 

 the syenite-granite series about the great anorthosite body shows little 

 evidence of having assimilated Grenville rocks. The Keene gneiss, par- 

 ticularly in the Lake Placid quadrangle, forms belts between the syenite- 

 granite series and the border (Whiteface) facies of the anorthosite as 

 well as between the syenite-granite and Marcy anorthosite. In both cases 

 the Keene gneiss exhibits essentially the same characteristics. Not every- 

 where does the Keene gneiss exist as definite zones or belts with syenite 

 or granite directly adjacent on one side and anorthosite on the other. 



How are these differences in distribution of the Keene gneiss to be ac- 

 counted for ? Also, why do the borders between the Grenville and syenite- 

 granite, as well as the Grenville and syenite-granite mixed gneisses, show 

 little or no evidence of magmatic assimilation? I believe the answer to 

 these questions may be found in the temperature relations of the rocks at 

 the time of the intrusion of the syenite-granite series. If v^e consider 

 that the great mass of anorthosite was still at a relatively high tempera- 

 ture, though not necessarily molten, it would have been only necessary 

 for the syenite-granite magma to have raised the temperature of the 

 borders of the anorthosite comparatively little to have effected actual 

 assimilation. 



XXXIV— Bull. Geol. Soc. Am., Vol. 20, 1917 



