
GEOLOGICAL SURVEYING 305 
occur. As the outcrops of thrust-planes usually follow the strike, they 
are sometimes hard to detect unless they be on a grand scale. But if 
the observer has been able to make out the general geological structure, 
and has ascertained that the schistose rocks show a more or less definite 
succession, careful mapping will reveal all the reversed faults of any 
importance. Frequently, indeed, these give rise to prominent features 
at the surface, following as they do some determinate direction athwart 
the face of mountain slopes, where they simulate the appearance 
of horizontal or inclined bedding-planes. Thrust-planes are usually 
rendered conspicuous by erosion. Naturally, they often bring into 
juxtaposition rocks of very different kinds—on one side, it may be, 
massive and relatively durable rocks ; on the other side, more readily 
disintegrated and degraded materials. Not infrequently, therefore, 
thrust-planes are laid bare by the removal of the softer rocks from 
the inclined surface of harder rocks upon which they rest. Or, in 
cases where a gently inclined thrust-plane has brought harder or 
more durable rocks to rest upon less resistant rocks, an escarpment 
may be developed by erosion, the geological structure producing the 
same effect as the intercalation of a relatively ““hard” bed in a series 
of “softer” strata. Occasionally, running water has hollowed out deep 
gullies and ravines along the outcrops of thrust-planes (Plate XLII.). 
The presence of a considerable thrust-plane is often further revealed 
by the crushed and brecciated appearance of the immediately adjacent 
rocks. So shattered may the rocks be, that the line of movement often 
resembles the outcrop of a breccia. Still more notable are the evidences 
of metamorphism induced by such great rock-displacements. Clastic 
rocks, for example, may be rendered crystalline and schistose, the 
foliation extending upwards for some little distance above the plane 
of rock-movement. Massive crystalline eruptive rocks may, in like 
manner, be crushed and foliated, while ancient gneissose and schistose 
rocks become similarly modified, new planes of foliation being developed, 
which may cross the older foliation at any angle. It is particularly 
noteworthy that the younger foliation always coincides in direction with 
the plane of rock-movement. 
The system of thrust-planes traversing schistose and other rocks in 
a region of highly complicated structure, is often cut across by one or 
more systems of normal faults, which shift the thrust-planes just as if 
they were outcrops. Such faults, therefore, can be detected and followed 
in the usual way. 
Archean Rocks.—If it be often a hard matter to unravel the structure 
of a region of highly metamorphosed rocks, it is still more difficult to 
map out the various complicated and puzzling phenomena presented by 
the ancient coarsely banded gneissose rocks that seem to form the 
foundation-stones upon which the oldest sedimentary strata of the 
globe have been laid down. Hitherto, all attempts to work out the 
structure and succession of the “Archzan complex,” as developed in 
particular regions, have been more or less unsuccessful. Now and 
again, what may at first appear to be a series of distinctive kinds of 
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