PETROGEAPHICAL OHARAOTEES OP UPPER HCTROlsIAN. 171 



The alteration of the feldspar usually begins at the peripher}^, and 

 gradually advances toward the center. It thus breaks the original grain up 

 into irregular areas and stringers of feldspar, many of which are attached to 

 the unaltered center. Between these residual areas of feldspar there are 

 irregular grains of secondary limpid feldspar and quartz. The farther the 

 alteration is advanced the less of the irregular center may be seen, and in 

 the final stage the feldspar core disappears. 



While the alteration nearly always begins at the periphery, one case 

 was noticed where it apparently began at various places in the. grain, the 

 result being the production of a secondary micropoikilitie structure. This 

 original feldspar is cloudy, with the usual alteration products, but scattered 

 through this are a number of more or less roundish spots of quartz, the 

 majority of which extinguish simultaneously, and have a different position 

 of extinction from the including feldspar. Considering these two elements 

 alone, the structtire is near the micropegmatitic ; but there are other areas 

 which extinguish in different positions from both the quartz and the original 

 feldspar. These are of decidedly more angular shape than the secondary 

 quartzes, and appear to be secondary acid feldspar. The small size of these 

 secondary minerals prevents the use of any physical tests other than the 

 differences in refraction. The rounded quartz appears in many respects 

 very much like the corrosion quartz of the French petrographers. The 

 majority of the secondary feldspars are unstriated, but a few show striations. 

 No satisfactory sections upon which to make measurements were found. 

 Biotite and nmscovite flakes are included in the quartz, and smaller auto- 

 morphic plates of biotite may be seen lying partly within the altered feld- 

 spar grain, as though growing partly at its expense. 



These highly metamorphosed micaceous rocks included under the gen- 

 eral term "micaceous graywackes" have the interlocking groundmass struc- 

 ture of the schists, but some of the larger grains show clastic forms. No 

 sharp line can be drawn between these metamorphosed sediments on the 

 one hand and the mica-schists and mica-gneisses on the other. 



In the mica-schists and mica-gneisses all of the original mineral grains 

 have been completely crushed and recrystallized, and we can find no micro- 

 scopical critei'ia which enable us to class them with the sedimentary rocks. 

 Dynamic action in the district had sufficient power and duration to complete 

 locally the metamorphism of the original sedimentaries and produce per- 



