MOUNT GREYLOCK. 183 



of chlorite and grains of quartz. Whether the hydrous character of the 

 rock proceeds from the chlorite or from some other hydrous mica can 

 hardly be determined, as the two minerals are intimately interlaced. The 

 talcose appearance and touch of much of the Greylock schist, which 

 have given it the names of talcoid-schist, hydro-mica schist, magnesian slate, 

 is due largely to the presence, almost if not quite universal, of these exceed- 

 ingly minute folia of chlorite; 1 and the variable proportions of the chlorite 

 and the muscovite in different localities explain the difference in the chem- 

 ical analyses of it as well as the variety of names geologists have given it. 

 The color of these schists varies with the varying proportions of its prin- 

 cipal ingredients — muscovite, chlorite, and quartz. Often it is black from 

 the presence of graphite, or porphyritic from the presence of feldspar, or 

 spangled from the presence of other minerals. Quartz lenses and seams are 

 almost universal. There are also great variations in the texture of these 

 rocks. Their structural peculiarities have been described at length on 

 pages 138-157, and constitute one of their chief characteristics 



The following is a brief summary of Mr. Wolff's microscopic analy- 

 ses of the typical specimens collected: Among the minerals of most fre- 

 quent occurrence are black tabular rhomboidal crystals or lenticular plates 

 of ilmenite and chlorite, a plate of ilmenite being interleaved between two 

 of chlorite. "Similar forms have been described by Renard from the met- 

 amorphic rocks of the Ardennes, but they are surrounded by sericrte layers 

 and not by those of chlorite." He also describes large plates of chlorite 

 inclosing small octahedra of magnetite, which also occur on Greylock. 2 

 Very minute bluish green crystals resembling the ottrelite of the Rhode 

 Island Coal-measures are found. 3 



Perhaps fully as common, if not more so, is albite, which occurs in 

 simple twins or untwinned, sometimes with a rim of clear feldspar separated 

 or not from the central crystal by a rim of quartz, and surrounded by 

 fibers of muscovite and chlorite. (Thus specimens from locality 458, south 



'See E. Hitchcock, Report Geol. of Vermont, 1861, vol. 1, p. 50+. James D. Dana, Am. Jour. 

 Sci., 3d ser., vol. 4, p. 366, and vol. 14, p. 139. 



- See A. Renard, Recherches sur la composition et la structure des phyllades ardennais. Bulletin 

 Mus. Roy. Belg., vol. 2, 1883, p. 127-152, and vol. 3, 1885, p. 230-268. 



'See J. E. Wolff: < >n some occurrences of ottrelite aud ilmenite schist in New England. Bull. Mas, 

 Comp. Zool., Geological Series, vol. 2, p. 159, 1890. Cambridge, Mass. 



