376 



QUARTZ ROCK AND QUARTZITE. 



Fig. 74- 



ciently parallel to give the idea of disturbed surfaces of deposition ; 

 on a small scale, by close examination, innumerable centres of local 

 forces, producing minute, recurring, and anastomosing curvatures, appear. 

 These minute flexures are due, not so much to general or 

 external pressure of the whole mass, as to the mechani- 

 cal displacements effected in the mass by the generation 

 of new minerals, as garnet, or the aggregation of others, as 

 quartz, or felspar, or both. Thus the mica and chlorite 

 which generally meet the surfaces of lamination appear to 

 have been shouldered about without being fused, twisted 

 in their structural planes, and subject to that curious 

 minute folding which is often observed as one of the 

 effects of cleavage structure in delicate and pliable shells, 

 in slates, for which the term " creep " was introduced by Professor 

 John Phillips (fig. 74). 



Accessory Minerals in Mica Schist. Various minerals are dis- 

 seminated through it, as garnet, which may be two inches in diameter 

 or almost invisible. It is common in the Alps, abundant in Bohemia, 

 rare in the Thuringerwald, and less rare in Scotland and Ireland. 

 Emerald, beryl, disthene, tourmaline, felspar, epidote, hornblende, 

 graphite, staurolite, chlorite, talc, fluor spar, baryta, are sometimes 

 abundant. The accessory minerals give names to many local varieties 

 of mica schist, such as garnet mica schist, chlorite mica schist, tour- 

 maline mica schist, hornblende mica 

 schist, &c. ; and sometimes the ac- 

 cessory becomes the typical or 

 dominant mineral. Molybdena, 

 rutile, oxide of tin, wolfram, 

 oxide of iron, grey cobalt, native 

 gold occur. Its metallic veins are 

 of the same nature as those in 

 gneiss. 



Physical Relations of Mica 

 Schist. Mica schist alternates in 

 the same way with quartz Tock, 

 hornblende schist, and the older 

 slates, and encloses similar deposits of limestone and dolomite. It 

 seems, therefore, almost superfluous to say that the line of rigid dis- 

 tinction between mica schist and gneiss cannot be drawn in the field. 

 On a great scale the two rocks retain their typical characters over large 

 tracts of country, and their distribution is hence essentially the same. 



Quartz Rod: 



Quartz rock, in the greater number of instances, especially when 

 occurring in veins, seems more recent than mica schist and gneiss, though 

 by easy changes in composition it becomes nearly identical with them. 

 The internal evidence of texture seems to decide the question of the 

 origin of quartz rock, and to prove that, however altered by subsequent 



Fig. 75- Mica Schist, q quartz, c chlorite. 



