36 XEW YORK STATE MUSEUM 



richly garnetiferous streaks. The rock has been highly sheared but 

 during the process, as the microscope shows, recr^'stallization has 

 predominated over granulation: or. the shearing was ettected in two 

 stages, hrst while the formation was buried in the zone of rock 

 tiowage : second, after erosion had brought the recrystallized mass 

 into the zone of fracture, at which time the application of pressure 

 resulted in partial granulation of the formation. Zircon, leucoxene 

 and magnetite ( the latter occasionally including pyrite ) are the ac- 

 cessories. The second locality is a short distance due north of the 

 east end of the crossroad running eastward from Beach Plains 

 Church. The appearance of the rock in the ledge is shown in plate 5, 

 lower hgure. which is very similar to the occurrence just described. 

 The garnets are ordinarily reddish and skeletal, and seldom more 

 than three-eighths to one-half of an inch in diameter. They are 

 abundant in the gneiss and are scattered in irregular streaks through- 

 out the ground mass. On a fresh surface they do not materially 

 attect the grayish or bluish gray color of the rock, which is due to 

 the free admixture of biotite flakes with the grayish feldspar. The 

 streaks which appear white in the illustration are of pegmatite, which 

 characteristically accompanies this formation in the sigmoid area, 

 but is generally absent from the garnet gneiss elsewhere. 



A thin section of the ledge rock of the 800-foot hill just west of 

 the Russell-Potsdam town line, near the margin of the map, shows 

 skeletal garnets, quanz. orthoclase and microcline with interstitial 

 biotite shreds. Straining and granulation are shown, but micro- 

 augen structure is rare. Banding is due to alternating quartzo- 

 f^ldspathic and biotitic streaks, the fomier of which sometimes 

 carry spindle growths of quartz. The garnet gneiss of the north 

 limb of the fold, as represented by some prominent outcrops near 

 the middle of the northwest edge of the area mapped, differs from 

 those described only in the presence of oligoclase-feldspar. in addi- 

 tion to orthoclase and microcline. and in the prominent granulation 

 shown in the mosaic texture. 



The mineralogical composition of the purer phases of the sigmoid 

 garnet gneiss is thus seen to be essentially like the other true garnet 

 gneisses already described. At other points, as. for example, back 

 of the house just south of the fork in the road which leads south- 

 ward out of Pierrepont, nongametiferous phases are developed. 

 the rock being reduced locally to a quartz-orthoclase-biotite gneiss. 

 As a rule, such masses are of small extent. In view of the fact that 

 in the garnet gneiss itself, the garnets are often restricted in their 

 distribution to streaks a few inches broad, with inter\-ening barren 

 areas a foot or more thick, it is impossible in mapping to distinguish 



