
FAULTS 171 
however, does actually occur in certain regions of highly 
folded strata. Faults of this kind are steeply inclined or 
vertical, and often extend for many miles, always traversing 
the strata at approximately right angles to the strike. They 
are neither downthrows nor upthrows, movement having 
taken place in a forward direction, so that the walls of the 
thrusts are slickensided horizontally. Not infrequently the 
rocks on either side are much crushed and shattered, or the 
two walls may be separated by a “friction-breccia.””. Among 
the best-known examples of such transverse thrusts or trans- 
current faults are those met with in the Alps and the Jura and 
in the Scottish Highlands. 
Origin of Faults.—Although we have yet much to learn as to the 
origin of faults, there are certain conclusions which seem fairly well 
established. From our descriptions of the phenomena of normal faults, 
the student has doubtless gathered that these displacements are most 
satisfactorily explained by the view that they are true downthrows and 
not upthrows. Their somewhat constant relation to the dominant folds 
of a region seems to suggest that they are, like many joints, the result 
of torsional strain. We may suppose that the rents were produced or 
commenced at the time the strata were being folded. But folding 
implies, of course, lateral compression, and it does not seem likely, 
therefore, that the fractured rocks could subside so long as compression 
continued. When this movement had ceased, however, the bulged-up 
crust, relieved from lateral pressure, would tend to sink again, and 
subsidence would naturally take place along the cracks and fissures 
which had already come into existence. The whole area we may think 
of as being split up into a series of rectangular blocks of varying size, 
each block defined by fissures, some of which would be inclined in one 
direction and some in another. In this way a long rectangular block 
defined by two parallel strike-faults inclined towards each other, would 
have a relatively narrow base; while the adjoining block, defined by 
two parallel strike-faults inclined away from each other, would have a 
relatively broader base. Thus, when gravitation came into play and the 
fractured rock-masses commenced to sink, those with a relatively narrow 
base (presenting as they would a smaller area to pressure) would tend 
to sink more readily than the broader based segments that adjoined 
them. In short, downthrow would take place in the direction of the 
hade. It must be noted, however, that some amount of lateral pressure 
would be exerted by the several subsiding masses, which might now and 
again result in local distortion, crushing, and fracturing, such as so 
frequently accompany normal faults. This lateral pressure would aiso 
account for the fact that now and again the rocks on both sides of these 
faults are turned or bent upwards or downwards (see Fig. 50). 
Apart, however, from any theoretical explanation of normal faults, 


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