


298 STRUCTURAL AND FIELD GEOLOGY 
shattered and traversed in all directions by irregular joints, 
the faces of which will frequently show slickensides and be 
coated with mineral matter. In short, veins of quartz, calcite, 
etc., may ramify more or less abundantly through the disturbed 
rock-masses. When the strata on both sides of the inferred 
fault are mapped, the observer will most likely find that the 
two sets of rock “strike at” each other—the outcrops of one, 
or it may be of both series, being truncated, as shown in the 
accompanying ground-plan (Fig. 117). The determination 
of the downthrown side of a very large fault is seldom 
doubtful. If the rela- 
tive age of the strata 
on. either side) of mime 
dislocation be known, 
and this is usually the 
case, the younger rocks 
will, of course, occur on 
the downthrow side. In 
cases where faults tra- 

FIG. 117.—FAULTED STRATA STRIKING 
AT EACH OTHER. verse one and the same 
series of rocks, andmare 
not exposed anywhere in section, the downthrow side of a 
dislocation will yet be rendered evident by the effect produced 
upon the outcrops, as already described (Chapter XI). 
Reversed Faults.—Reversed faults occurring amongst 
strata the geological age of which is known are not hard 
to detect. When Carboniferous rocks, for example, are seen 
dipping regularly underneath Devonian beds, it is obvious 
that this inversion of the stratigraphical succession must be 
due either to overfolding, or to an overthrust, or to a com- 
bination of both structures. If the inversion be the result 
of folding alone, then it is obvious that both series of rocks 
occurring in the reversed limb of a strongly unsymmetrical 
or recumbent fold must be turned upside down. If, on the 
other hand, the inversion has been caused by a reversed 
fault alone, then there will be no overturning of the beds in 
either series of strata, the individual beds of the Carboniferous 
will occur in regular sequence, and the same will be the case 
with the Devonian strata. But, as reversed faults have fre- 
quently resulted from the yielding of unsymmetrical folds, it 
