28 
SCENERY OF SWITZERLAND. 
covered by several thousand feet of rock, all of 
which have been removed. 
“Probably,” says Geikie, “the great majority of 
geologists now adopt in some form the opinion, that 
the oldest or so-called ‘Archaean’ Gneisses are 
essentially eruptive rocks Whether they were 
portions of an original molten ‘magma’ protruded 
from beneath the crust or were produced by a re- 
fusion of already solidified parts, of that crust or of 
ancient sedimentary accumulations laid down upon it, 
must be matter of speculation.” * 
On the other hand, Gneiss is certainly not all of 
the same age, since in some instances it traverses 
other strata. There appear, moreover, to be cases 
in which sedimentary strata have been metamorphosed 
by heat or pressure into a rock which cannot minera- 
logically be distinguished from Gneiss. 
Gneiss presents many varieties. The principal 
are Granite -gneiss, where the schistose arrangement 
is so coarse as to be unrecognisable, save in a large 
mass of the rock; Diorite-gneiss; Gabbro-gneiss, com- 
posed of the materials of a Dolerite or Gabbro, but 
with a coarsely schistose structure; Porphyritic-gneiss 
or Eyed-gneiss, in which large eye-like kernels of 
Orthoclase or Quartz are dispersed through a finer 
^ Text-book of Geology. 
