440 W. J. MILLER ADIRONDACK AXORTHOSITE 



ciated with 10 to 15 per cent ferro-inagnesian minerals. The syenite is 

 quite normal in every respect except that it is a little finer grained than 

 usual. A thin section shows the following percentages of minerals: 

 Microi^erthite, 64; orthoclase^ o; oligoclase, 20; quartz, 6; hornblende, 

 2V2j and other very minor constituents. 



Most of the rock of the ledge, however, is clearly an assimilation prod- 

 uct of syenite and anorthosite. This rock (Keene gneiss) exhibits at 

 least three distinguishable facies. One 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 number 

 42 of the above table. A second facies is only faintly gneissoid, with lab- 

 radorite phenocrysts only roughly parallel to the foliation. Its composi- 

 tion is given as number 45 of the table. The j^resence of orthoclase and 

 a greater amount of microperthite makes this rock more syenitic than the 

 first facies. In the two facies just described the phenocrysts of labra- 

 dorite not only finely exhibit polysynthetic twinning, but they are also 

 perfectly and conspicuously twinned according to the albite law, thus giv- 

 ing the freshly broken surface a striking appearance. Both of these 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 non-foliated and contains no 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 each other and they are quite 

 certainly only phases 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 side and the true anorthosite on the other, but rather by narrow 

 transition zones. All three facies of this Keene gneiss are certainly inter- 

 mediate in composition between the syenite and anorthosite, the first one 

 described having decided anorthosite affinities, the third having decided 

 syenite affinities, and the second being no more syenitic than anorthositic. 

 The conclusion, therefore, based on the field relations and composition of 

 the rocks, is that we have here a true magmatic assimilation product, the 

 invading syenite magma having actually incorporated and digested more 

 or less of the anorthosite material. 



The close juxtaposition of the syenite and Keene gneiss may be reason- 

 ably explained by considering the syenite to have been an intrusive off- 

 shoot from the near-by large body of syenite into previously formed and 

 cooling, or possibly solidified, Keene gneiss magma, the temperature then 

 having been high enough only to permit fusion along a narrow border 



