GEOLOGY OF THE SCHROON LAKE QUADRANGLE 5I 



site, and that the Keene gneiss was not formed as a differentiate 

 in situ between an overlying sheet of syenite-granite magma and 

 underlying anorthosite. 



Significance of distribution of Keene gneiss. The Keene gneiss 

 can not be a direct differentiate of either the syenite-granite series 

 or the anorthosite because it never occurs except on the border, 

 or close to the contact, between the syenite or granite and the 

 anorthosite. If we make the very simple and plausible assumption 

 that the anorthosite was still very hot when the syenite-granite 

 magma was intruded, or, in other words, if this latter magma was 

 forced up comparatively soon after the development of the anor- 

 thosite, 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. But the Keene gneiss is not universally present. In 

 many cases where no Keene gneiss occurs along the borders between 

 anorthosite and syenite or granite, it may be reasonably assumed 

 that either the anorthosite or the syenite-granite, or both, in those 

 places may not have been hot enough to permit assimilation. 



The presence of Keene gneiss in. one place and its absence from 

 the same border nearby, may, in some cases, have been the result 

 of unequal upward intrusion of Keene gneiss magma which orig- 

 inated at lower levels. 



The small isolated masses of Keene gneiss some distance out 

 from the main body of the anorthosite doubtless represent inclusions 

 of anorthosite which were partly or completely assimilated by the 

 enveloping syenite or granite magma. 



The failure to find any 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 permitted any more than compara- 

 tively slight assimilation by the invading syenite-granite magma. 

 It should be borne in mind, however, as pointed out in a recent 

 paper^ by the writer, that local assimilation of the Grenville is 

 known to have taken place in certain parts of the Adirondack 

 region. 



Anorthosite and Syenite-granite Mixed Rocks 



A very irregular-shaped area of about 5^ square miles, includ- 

 ing Hayes mountain, is represented on the map as anorthosite and 



^ Geol. Soc. Amer. Bui., 25 :254-6o. 1914. 



