50 NEW YORK STATE MUSEUM 



of the syenite-granite series. If we consider that the great mass of 

 anorthosite was still at a relatively high temperature, though not 

 necessarily molten, at the time of the syenite-granite intrusion, it 

 A'ould have been only necessary for the syenite-granite magma to 

 have raised the temperature; of the borders of the intruded anor- 

 thosite comparatively little to have effected actual assimilation. 



The tongues of syenite cutting Whiteface anorthosite on Wil- 

 mington mountain, and the tongues of granite cutting Whiteface 

 anorthosite on Mt Whiteface (described on page 33), furnish 

 important evidence in support of this view, because these tongues 

 or dikes, instead of being in real sharp contact with the anortho- 

 site, show very narrow transition zones due to slight fusion of the 

 anorthosite. Now, it does not seem at all probable that even small 

 amounts of relatively cold anorthosite could have been fused and 

 assimilated by such small masses of intrusive magma, but with the 

 anorthosite at a high temperature, though not really molten, its 

 borders might very conceivably have been fused. Thus, if we make 

 the very simple and plausible assumption that the anorthosite was still 

 very hot (though not necessarily molten) when the syenite-granite 

 m.agma was intruded, or, in other words, that this latter magma was 

 forced up comparatively soon after that of the anorthosite, the 

 usual strong objection to magmatic assimilation, namely, that a 

 magma does not possess a sufficiently high temperature to raise 

 relatively cold country rock to the point of fusion, is distinctly 

 obviated. 



Where no Keene gneiss occurs along the borders, it may be 

 plausibly conceived that either the anorthosite, or the syenite- 

 granite, or both, may not have been hot enough to permit assimila- 

 tion. Also, in harmony with this hypothesis, the failure to find an}' 

 considerable assimilation of Grenville either along its borders with, 

 or where involved with, the syenite-granite series may be explained 

 on the basis of a temperature of the Grenville too low to have per- 

 mitted any more than comparatively slight assimilation by the 

 iuA^ading syenite-granite magma. It should be borne in mind, how- 

 ever, as pointed out in a recent paper by the writer, that local 

 assimilation of Grenville was not uncommon in the Adirondack 

 region.^ j ,' ; i g j 



In the mixed rock areas near the center of the quadrangle, where 

 anorthosite has been cut to pieces by intrusions of syenite, the few 

 contacts observed are not very sharp. Apparently, in these areas 



* Geol. Soc. Amer. Bui. 25 1254-63. 1914. 



