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ALTERATION AND METAMORPHISM 227 
sometimes to the strains and stresses due to crustal movements. 
Occasionally thermal and dynamo-metamorphism may have acted to- 
gether, and in such cases it may be impossible to say which of the two pro- 
cesses has played the dominant réle. In both pressure is recognised as 
an important factor, and the presence of water, either in the liquid or 
the gaseous form, is another essential condition—water itself acting asa 
mineralising agent and carrying with it various other chemical agents of 
change. But the phenomena of slaty cleavage, and the cataclastic struc- 
tures so frequently met with amongst crystalline schists are clearly the 
result of compression and crushing, and can only be explained by the 
theory of dynamo-metamorphism. Even schistose structure on the large 
scale can hardly be accounted for without pressure. The metamorphism 
of rocks, however, is still far from being satisfactorily explained. 
Geologists are much divided in opinion, and many observations and 
much research, as well chemical and physical as geological, will be 
required, before an adequate conception of the subject can be attained. 
Archzan Rocks.—Under this head are included a remarkable group 
of coarsely crystalline gneissose rocks, the origin of which has been a 
fruitful subject of discussion. The rocks in question, although termed 
gneiss, are not truly schistose rocks. They show a banded structure, 
indeed, but this cannot be confounded with foliation, but is suggestive 
rather of a kind of fluxion-structure. The bands in question somewhat 
resemble those streaky layers and veins so commonly present in certain 
massive eruptive rocks. The constituent minerals of the layers referred 
to seem to have segregated either while the igneous rock was in motion 
or after it had ceased to move. The gneissose rocks, moreover, ever 
and anon lose their banded structure, and merge into massive rocks, 
which cannot be distinguished from granitoid eruptives. Not only so, 
but they frequently behave as intrusive rocks, one gneiss cutting across 
another. These and other appearances lead to the belief that the 
Archean granite-gneisses are of igneous origin. They underlie the 
oldest stratified rocks of the globe, wherever the base of these is exposed, 
and hence are thought by some geologists to represent the original crust 
formed upon the surface of the globe. They vary much in composition— 
from highly acid to highly basic. In some places they appear to 
alternate with truly schistose rocks and crystalline limestones, as if 
all belonged to one and the same series. This appearance, however, is 
perhaps deceptive, and due to the intrusive character of the gneisses. It 
may be added that almost everywhere the Archzean rocks yield evidence 
of having been subjected to powerful deformation—they have frequently 
been crushed, pulverised, recrystallised, and foliated. 
