448 W. J. MILLER— ADIRONDACK ANORTHOSITE 



The tongues of syenite cutting Whiteface anorthosite on Wilmington 

 Mountain, and the tongues of granite cutting similar anorthosite on the 

 side of Mount Whiteface (see figure 1), furnish important evidence in 

 support of this view, because these tongues or dikes, instead of being in 

 sharp contact with the anorthosite, show very narrow transition zones due 

 to slight fusion of the anorthosite. Now it does not seem probable that 

 even small amounts of comparatively 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, the 

 borders might very conceivably have been fused. Thus, 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 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 assimilation. In this connection, it 

 should be noted that the prominent mass of syenite-granite which projects 

 for many miles into the anorthosite of the Lake Placid quadrangle (see 

 above) shows little or no development of Keene gneiss along its borders 

 except well within the quadrangle, where it is reasonable to believe that 

 the anorthosite was hotter, due to greater thickness and slower cooling of 

 the laccolithic body there. In both the Schroon Lake and Long Lake 

 quadrangles, however, considerable developments of Keene gneiss took 

 place along or close to the outer margin of the great body of anorthosite, 

 probably because on the south and west sides the anorthosite of the lacco- 

 lith was notably thicker, and hence kept hot longer (see figure 3). 



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

 same border near by may in some cases have been the result of unequal 

 upward intrusion of Keene gneiss magma which originated at a lower 

 level. 



The failure to find any considerable assimilation of Grenville either 

 along its border 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 comparatively 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 

 was not uncommon in certain parts of the Adirondack region. 



** W. J. Miller : Bull. Geol. Soc. Am., vol. 25, 1914, pp. 254-260. 



