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properly mica gneiss — a rock only differing' from granite in 
possessing a foliated parallel structure : an attempt to give 
some idea of this structure is made in PI. LXXIII., fig. 5, which 
is taken from a piece of ordinary gneiss. The origin of gneiss is 
still involved in much obscurity, and there can be no doubt but 
that some varieties of gneiss have a totally different origin from 
others. It is known that beds of true mica or hornblende 
schist, i.e. those composed only of hornblende or mica along 
with quartz, often become felspathic close to their junction 
with eruptive granites, so as to become lithologically gneiss, 
although petrologically they can only be termed felspathic 
schists, since by following up these beds they are soon found to 
be true schists. 
Other and more characteristic gneiss, in which the felspathic 
element is inherent and much more prominent, has from old 
times, and with much show of reason, been regarded as formed 
from the debris of eruptive granites, due either to subaerial dis- 
integration or to their having been ejected into or under water, 
and thus converted into the condition of tuff, which, after 
having been arranged under water as sedimentary beds and 
consolidated, have become subsequently crystalline by meta- 
morphic action, thereby causing them to assume the foliated 
appearance they now present. 
Whatever may be the true origin of ordinary gneiss, there is, 
however, another variety of this rock, called granitic gneiss, 
which cannot have been other than eruptive granite originally, 
in which the parallel arrangement of foliation is a superinduced 
structure, developed in it subsequent to its solidification ; in 
fact it is a metamorphic eruptive rock, just as the ordinary 
schists are but metamorphic sedimentary beds. 
The proof that such granite gneiss was originally eruptive is 
seen in the disturbance which it has occasioned in the rocks 
through which it has broken, as also in the fragments of these 
rocks which it encloses ; thus PL LXXIII., fig. 3, is taken from 
Darwin’s admirable work on the Geology of South America,, 
and represents a fragment, 7 yards long by 2 wide, of dark 
coloured rock with garnets in it, enclosed in the ordinary 
granitic gneiss of the country about Eio Janeiro, both being cut 
through by a still more recent small granite vein ; fragments 
and patches of the other rocks are also seen in the granitic 
gneiss of Donegal and Galway in Ireland. 
A magnificent section, several mile$ in length, of such granitic 
gneiss can be seen in the naked and almost perpendicular cliffs 
on the north side of Eidsvand, a lake in the south of Norway : 
a portion of this section, elsewhere published by the author in 
1856, is represented in PI. LXXIII., fig. 1, and shows the original 
dark hornblendic schists of the country broken through and 
