GEOLOGY OF THE LAKE PLACID QUADRANGLE 45 



Areas in the vicinity of Keene village. The type locality of 

 the Keene gneiss is a ledge by the side of the state road at the north- 

 ern edge of the village of Keene where an excellent opportunity is 

 afforded for the study of the rock and its relations to both anor- 

 thosite and syenite. Ail three of these rocks show as unweathered 

 material in this one ledge which has been recently blasted open. 

 The anorthosite, which occurs in minor amount, is the typical 

 Alarcy facies consisting mostly of dark, bluish gray labradorite up 

 to an inch across embedded in some granulated feldspar, and 

 associated with lo to 20 per cent of ferro-magnesian minerals. 

 The syenite is quite normal in every respect except that it is a 

 little finer grained than usual. No. 24 of table 2 gives its mineral 

 content. Most of the rock of the ledge, however, is clearly an 

 assimilation product of syenite and anorthosite. This assimila- 

 tion rock (Keene gneiss) exhibits at least three distinguishable 

 facies. One of these is highly gneissoid with elongate cores of 

 labradorite crystals as phenocrysts up to an inch long arranged 

 parallel to a distinct foliation. Its mineral content is given as no. 

 42 of table 3. A second facies is only faintly gneissoid, with labra- 

 dorite phenocrysts only roughly parallel to the foliation. Its com- 

 position is given as no. 45 of table 3, the presence of orthoclase 

 and a greater amount of microperthite making this rock much more 

 ?yenitic than the first facies. In the two facies just described, the 

 phenocrysts of labradorite not only finely exhibit polysynthetic 

 twinning, but they are also perfectly twinned according to the 

 albite law, thus giving the freshly broken surface a striking appear- 

 ance. Both of the facies are notably granulated, and the rounded 

 phenocrysts are the uncrushed portions of what were once still 

 larger crystals. A third facies, in minor quantity, is nonfoliated 

 and contains no labradorite phenocrysts, but it does contain a few 

 rounded red garnets up to an inch across. This third facies is 

 the most syenitic of the three. 



All three facies just described grade into one another and they 

 are quite certainly only differentiates of a single cooling magma. 

 Also it is important to note that the Keene gneiss is not sharply 

 separated from the true syenite on one hand, and the true anortho- 

 site on the other, but rather by narrow transition zones. All three 

 facies of the Keene gneiss are certainly intermediate in composi- 

 tion between the syenite and anorthosite, the first one described 

 having decided anorthosite afitinities, the third having decided 

 syenite affinities, and the second being very clearly intermediate 



