GEOLOGICAL ANTIQUITY OF GNEISS. 573 



rocks of gneiss. The term subordinate, on a great scale, is not inv 

 properly applied to these lenticular masses, though in local geology 

 their occasional great extent and comparative regularity may entitle 

 them to be classed under an independent title. 



By the substitution of hornblende for mica, gneiss gradually 

 changes to hornblende schist ; the loss of its felspar approximates it 

 to mica schist, the diminution of its mica produces a resemblance to 

 quartz rock. A finely granular slate, with more evidence than usually 

 appears of watery friction among the particles, almost makes a transi- 

 tion from gneiss to sandstone, as at Dalnacardoch. A more minute 

 admixture of its ingredients, with a predominance of chlorite, gives it 

 the aspect of argillaceous slate. These gradations are observed most 

 frequently at the junctions and alternations of the several rocks. 



Gneiss forms Intrusive Veins. Yon Gotta remarks that it was 

 formerly believed that all gneiss is of metamorphic origin, but it has 

 been established that many kinds of gneiss are eruptive. In the min- 

 ing districts of the Erzgebirge there is a red gneiss containing 75 per 

 cent, of silica, and a grey gneiss containing 65 per cent, of silica ; and 

 this red gneiss or gneissite is said sometimes to form dykes and veins 

 in the common grey .gneiss. 



Grey Gneiss Associated with. Mineral Veins. The grey gneiss is 

 more frequently associated with metalliferous veins rich in silver, and 

 this relation has been attributed to the influence of the iron yielded 

 in greater quantity by the decomposition of the mica in the grey 

 gneiss. 



Geological Age of Gneiss. Gneiss commonly occurs beneath 

 the sedimentary rocks. It has hence often been regarded as the 

 oldest rock, and there is a disposition among geologists to refer any 

 gneiss beneath primary rocks to the most ancient or Archaean period 

 of Dana. But since slates occur of all geological ages, and granite 

 originated in every period of geological time, we are compelled to 

 believe that schists must have similar differences in antiquity. 

 But gneiss of these? several periods has not yet been determined, 

 because it is almost impossible to fix the age of the base of the 

 gneiss, though the age of the stratified rock above usually offers no 

 difficulty. On the Continent there has been an attempt to separate 

 an older and a younger gneiss, the older rock being beneath the 

 strata, and the younger gneiss resting upon them. But such an ano- 

 malous succession suggests the probability of its being due to some 

 grand inversion of the country ; though, since metamorphism may be 

 of any geological age, there is nothing impossible in gneiss of any 

 age resting upon any strata. 



Fundamental Gneiss. Among the Continental localities for 

 gneiss, Zirkel mentions the Saxon Erzgebirge. This rock near 

 Freiberg has been divided into an older grey gneiss free from inclu- 

 sions, and a younger gneiss, grey or red, which includes fragments 

 of the older gneiss and various slates. Much of Bohemia and 

 Moravia is formed of the younger gneiss. Similar rocks are seen 

 in the Sudetic mountains, the Eulengebirge, and Kiesengebirge, and 



