PETROGRAPHICAL CHAKAOTEK OF THE WEWE SLATE. 265 



there are usually alteniatiug- layers of finer and coarser material, \\liil(' in 

 the gneissoicl granite the lamiiue are all alike. Tlu' reconi[)Osed rocks 

 contain a much larger amount of secondary iron oxide than the' schistose 

 granites, and in the folding thev have been more hrokcn, thus producing 

 distinct cracks and minute canities, which have become filled with finely 

 crystalline, secondary quartz. Tlie thin section thus enables us to discrim- 

 inate the recomposed rocks from the original, mashed, gneissoid g-ranite. 

 Many of tlie feldspars of the quartzites are sufficiently fresh to show 

 distinctly their twinning, but all of them are more or less kaolinized. 

 Frequently the feldspars have largely or wholly decomposed into a com- 

 plex, interlocking, finely crystalline mass of sericite and (juartz, chlorite 

 and quartz, biotite and quartz, or comlnnations of these. In an interme- 

 diate stage there is with these residual feldspar. Often during or subse- 

 quent to this decomposition much secondary iron oxide has entered, and 

 in these cases we have in place of the feldsjjar grains an interlocking mass 

 of iron oxides, quartz, and sericite. 



By a change in the character of the groundinass and a decrease in 

 the size of the fragmeutal grains tlie (juartzites pass into the graywackes. 

 The same constituents are present in the groundmass of the latter as in 

 that of the quartzites, but chlorite is abundant, and intermingled with the 

 groundmass are very small fragmental grains of quartz and feldspar, and 

 frequently a large amount of secondary iron oxides, chiefly hematite and 

 magnetite, often with distinct crystal outlines. In sttme cases a film of 

 oxide of iron is around each of the individual grains of (][uartz. In the 

 background, as the rocks become mashed, the leaflets of sericite and 

 biotite have a tendenc}' to a parallel ai-rangement. The coarser (juartz 

 grains uniformly show undulatory extinction or fracturing, fre(|uentlv 

 according to a rectangular s\-stem. The smaller (juartz grains, where 

 buried in an abundant matrix, and therefore not pressed against one 

 another, are freer from these pressure eflects, and in some of them pressure 

 efi'ects are not seen at all. The (juartz grains are much more frequently 

 enlarged than in the basal quartzites. The feldspars, while often rather 

 fresh, show all phases of decomposition to sericite, biotite, or chlorite, and 

 to quartz with iron oxide impregnation, described in the conglomerates. 



