72 
OROGENY AND EARTH’S ROTATION 
ed upon merely as a structural feature its larger relations, as a 
tectonic feature, are mainly either completely ignored or misinter¬ 
preted. That this acquired geologic structure is really something 
more than an abnormal phenomenon is only beginning to be appre¬ 
ciated. A product wholly independent of gravity strains it is in 
its ultimate analysis the direct release of orogenic tension. It is 
not merely a fortuitous accompaniment of secular shrinking of 
the earth’s interior. 
Although, as a structural feature, the thrust-fault is a theme of 
wide study its relatively infrequent occurrence makes it a rather 
unfamiliar phenomenon. Compared with the normal fault, it is 
a giant affair. The general tendency to consider the two types 
together reverses their relative and real importance. It tends to 
magnify the small one and dwarf the large one. Beyond the ob¬ 
servation that the hade of the reversed fault possesses a large 
angle while that of the normal gravity-fault has small value, and 
that as their names signify, direction of movement in the two is 
directly opposite, the broader tectonic aspects avail little. 
Thus the larger aspects of thrust-faulting appear to be seriously 
neglected. That thrust-faults occur more frequently in the hard 
brittle formations than elsewhere, that they are, perhaps, mainly 
confined to the very old rocks, rather than to the younger ter- 
ranes, that they become more prominent, more important, and 
more numerous in the bottom of the earth’s crust than near the 
top, and that under ordinary circumstances they rarely reach sky 
except through chance exposure by profound erosion, are gener¬ 
alizations of great significance, albeit that they attract small notice 
and are little associated genetically. 
On the basis of direct telluric effect rather than of local struc¬ 
tural consequence inductive analysis of thrust-phenomena in the 
earth’s crust points at once to the advisability of more than simple 
orographic treatment. Recent geotectonic experimentation gives 
clue to the probable succession of events in nature, and elucidate 
many of the various peculiarities of thrust-faulting heretofore per¬ 
fectly inexplicable. 
If it be premised for the earth’s crust, or zone of rock-fracture, 
a straticulate structure of heterogeneous composition, stresses of 
unequal moment are set up by normal rotation. The entire prism 
down to the zone of rock-flow cannot act therefore as a unit. 
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