70 NEW YORK STATE MUSEUM 



and fitful variation in the proportion of the three constituent rock 

 types. Such irregularity, indeed, has rendered detailed and accurate 

 mapping of the individual components practically an impossibility on 

 the small scale adopted. 



As regards mineralogical constitution, there is no great or essen- 

 tial difference between the injected granites of the sigmoid area and 

 the bulkier masses of the western part of the quadrangle. A speci- 

 men of a nearly massive pink granite from the southwest corner of 

 the area was seen under the microscope to be made up almost ex- 

 clusively of quartz, orthoclase and oligoclase, with subordinate mi- 

 croperthite, and an occasional hornblende or biotite xenocryst. The 

 rock appears to the eye to be as free as may be from impurities of 

 an included nature; nevertheless the presence of the ferromagne- 

 sian minerals amphibole and mica indicate the slight contamination 

 of the magma with nearly or wholly digested material of xenolithic 

 origin. Such basic constituents are not essential to the composition 

 of the granite, which is seen in numerous other thin sections to be 

 quite devoid of the femic minerals. In one other respect the example 

 cited departs slightly from the normal, namely, in the inferior de- 

 velopment of microcline. This mineral in the typical or ideal gran- 

 ite has a prominence probably nearly equal to that of the orthoclase 

 or plagioclase; and in the gneisses of the western half of the sheet it 

 holds about equal sway with the microperthite. A composite speci- 

 men would show some such mineral association as the following: 

 quartz, orthoclase, oligoclase-andesine, microcline, microperthite, 

 with magnetite, apatite and zircon as accessories, but in reality the 

 last two feldspars are seldom if ever seen in the same section. 



The quartz, except in the rare massive phases of the rock, oc- 

 curs in long, irregularly anastomosing spindle-shaped streaks, as is 

 shown in unusual development in plate 12, upper figure, which is a 

 photomicrograph of a Stengel gneiss north of Pyrites, near the head 

 of the reentrant in the granite boss. The plagioclase and feldspar, 

 under the conditions of dynamic metamorphism which induced a 

 total recrystallization of the free quartz, have yielded to the pres- 

 sure by mechanical deformation; optical disturbances in the 

 plagioclase are not common in the granite areas west and south of 

 Canton, but in the granite gneisses of Pyrites vicinity and through- 

 out the sigmoid belt, bending of the twinning lamellae and shadow 

 strains in the untwinned feldspar are some of the most character- 

 istic features of the rock. 



