372 MODES OF OCCURRENCE OF GNEISS. 



gneiss. This is seen in quarries of the stony hill of Nigg ; the gneiss 

 is fine grained, plicated, and darkly striped ; its layers gradually pass 

 into granular dark grey granite. Fragments of gneiss are often found 

 in the Aberdeen granite, and Heddle regards the kidney-shaped inclu- 

 sions, rich in black mica, as portions of gneiss. 1 



Bedding of Gneiss. Beds of gneiss are of various thicknesses, 

 and the laminae of which they consist are subject to such extra- 

 ordinary curvatures, that it is often difficult to trace them. 



Where gneiss alternates with other rocks, such as hornblende slate, 

 quartz rock, limestone, or mica slate, the stratification is rendered 

 very evident, but otherwise the beds are less regular, and often dis- 

 continuous. 



The contortions of the laminae of gneiss are most numerous and 

 surprising where, as frequently happens, veins of granite, quartz, 

 or felstone divide the gneiss. These veins cross the laminae at 

 various angles, and generally cause some peculiar twists along their 

 sides ; veins not infrequently insinuate themselves between the 

 laminae, and in this case, when thick and extensive, may be mis- 

 taken for alternating strata. Some cases of supposed alternation 

 between gneiss and granite may be thus explained, and in other cases 

 the rock called granite may be really a coarsely-granular gneiss. 



Minerals in Gneiss. Gneiss being one of the most widely-distri- 

 buted foliated rocks, is a rich repository of minerals, both in the 

 Old World and America. Garnets frequently, zircon, beryl, disthene, 

 epidote, tourmaline, rutile, oxide of tin, oxide of iron, sulphide of 

 molybdena, more rarely, are disseminated in its laminae. The veins 

 of quartz, calcareous spar, carbonate of iron, and sulphate of baryta, 

 which divide it, contain the sulphides of lead, copper, and zinc, 

 native silver, tin, &c., in Sweden, Germany, and Brazil; and many 

 other minerals occur in the calcareous strata which alternate with or 

 are enveloped by layers of gneiss. We can only suppose that the 

 metallic substances diffused in the sea were extracted by organisms, 

 or precipitated with the sediments, and that the minute and diffused 

 particles became collected by solvent water under the influence of 

 metamorphism. 



Rocks Associated with Gneiss. Gneiss alternates with granite 

 in the Riesengebirge and in Quito, and in some cases graduates into 

 the character of granite, as on the southern declivity of the Titlis and 

 Jungf rau (the age of this gneiss, however, may be more recent) ; more 

 frequently it exchanges beds with mica schist, hornblende schist, 

 and granular limestone and clay slate. These rocks are sometimes 

 in such small quantity as merely to mark lines of division in the 

 mass of gneiss, but at other times they swell out to great thickness. 



The limestone beds in particular are remarkably local and irregular 

 in their occurrence, and instead of extending, like the more recent 

 calcareous strata, through large tracts of country, appear in the form 

 of large lenticular masses, enveloped on every side by the predominant 



1 Heddle, Trans. Roy. Soc. Edin., vol. xxix. p. 6; see however supra, p. 218 

 and p. 259. 



