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306 STRUCTURAL AND FIELD GEOLOGY 
gneiss, alternating the one with the other, seems to suggest a possible 
chronological succession. But this apparent order rarely continues for 
any distance. Frequently, one of the gneissic bands will break across 
another—while the evidence of extreme deformation is everywhere 
conspicuous. The belief is gaining ground that these ancient rocks 
are probably for the most part of deep-seated igneous origin—comparable 
to those batholiths of granite, etc., with their associated sheets, dykes, 
and veins, which have given rise to the phenomena of contact meta- 
morphism. For sheets of coarsely banded gneiss not only cut across 
similar sheets of gneiss and beds of what seem to be metamorphosed 
sedimentary rocks, but ever and anon the gneisses lose their banded 
structure and graduate into amorphous granitoid masses. Since the 
time or times of their extrusion, however, all these igneous rocks have 
been subject to repeated deformation, and dynamo-metamorphism has 
modified them more or less profoundly. 
Although the rocks in question are usually grouped under the general 
term ‘‘Archzean,” it is by no means certain that they all belong to early 
pre-Cambrian times. It is quite possible that, in some cases, they 
may represent metamorphosed sediments of early Palzozoic age, pierced 
in all directions by masses and sheets of intrusive eruptives. It is 
obvious, indeed, that unless they are immediately overlaid by rocks of 
Cambrian age, their pre-Cambrian origin cannot be demonstrated. 
Nevertheless, the general similarity of the rocks of the so-called “‘ Archzean 
complex” all the world over, is suggestive. But the mapping of these 
old gneissoid rocks, and the interpretation of their evidence, are beyond 
the resources of the beginner. Not without much patient and skilful 
work in the field, and prolonged investigation in the laboratory, will the 
Archean rocks give up their secret. 
