3/0 NATURE AND ORIGIN OF GNEISS. 



Gneiss. 



Gneiss: its Origin. That the materials of the mechanically aggre- 

 gated gneiss rocks are derived from the disintegration of more ancient 

 granite and other crystallised compounds, is an opinion which is 

 strongly suggested by examining the composition of gneiss. 



The component minerals of gneiss and granite are the same 

 quartz felspar, and mica. They are mixed with the like accidents and 

 permutations, and occasional admixture of other minerals; and are 

 subject in both rocks to the same extreme variation of size. But these 

 rocks differ in the mode of arrangement among their constituent 

 minerals. 



The ingredients of granite are so connected together by con- 

 temporaneous or nearly contemporaneous crystallisation, that one 

 mineral penetrates and is intimately united with another ; and we are 

 compelled to conclude that they were not accumulated in distinct crystals 

 ready formed, but that the minerals never had a separate existence as 

 solids until their different geometric forms were slowly developed by 

 crystallisation. 



Gneiss almost always suggests, by some degree of imperfection 

 of the edges and angles of the quartz and felspar, and much more 

 decidedly by the laminar arrangement of the mica and consequent 

 minute foliation of the rock, that its materials, ready made and 

 crystallised, were brought together and arranged by water. Gneiss 

 in all its gradations, from a rock resembling granite to a fine-grained 

 fissile mass hardly distinguishable from clay slate, may be compared 

 with a series of sandstones, many of which are composed of granitic 

 detritus, and strongly allied to it in structure, but not having 

 undergone metamorphism, show unobliterated evidences that they 

 were aggregated by water. Gneiss, however, is not always the 

 crystallisation of materials in the layers in which they were aggregated, 

 but more frequently a chemical integration of minerals under the sol- 

 vent influence of water contained in the rock which was transformed ; 

 and this water first dissolved the constituents of the minerals, and 

 then often became included in the minerals themselves. 



In a great majority of instances, gneiss immediately rests upon 

 granite ; and we may suppose that the two rocks are effects of the 

 actions of pressure and heat under different conditions. 



Mineral Constituents of Gneiss. Gneiss is essentially a mass of 

 quartz and felspar, foliated with thin films of mica which are some- 

 times exposed by fracture. As in granite, the felspar is usually ortho- 

 clase, but oligoclase is sometimes associated with the orthoclase, though 

 oligoclase is more frequent in hornblende gneiss and protogine gneiss ; 

 there are varieties of gneiss in which orthoclase is the only fel- 

 spar. Occasionally albite is associated with orthoclase. Orthoclase 

 varies in colour in gneiss quite as much as in granite, and is sometimes 

 found in porphyritic crystals. The quartz occurs either in grains or 

 small lenticular plates made up of many crystals united together. The 



