PARALLEL FOLDS OF STRATA. 
93 
certain linear tracts. And marine denudation, to be treated 
of in the next chapter, will help us to understand why that 
which should be the protruding portion of the faulted rocks 
is missing at the surface. 
Arrangement and Direction of Parallel Folds of Strata. —The 
possible causes of the folding of strata by lateral movements 
have been considered in a former part of this chapter. No 
European chain of mountains affords so remarkable an illus¬ 
tration of the persistency of such flexures for a great dis¬ 
tance as the Appalachians before alluded to, and none has 
been studied and described by many good observers with 
more accuracy. The chain extends from north to south, or 
rather N.N.E. to S.S.W., for nearly 1500 miles, with a 
breadth of 50 miles, throughout which the Palaeozoic strata 
have been so bent as to form a series of parallel anticlinal 
and synclinal ridges and troughs, comprising usually three 
or four principal and many smaller plications, some of them 
forming broad and gentle arches, others narrower and steep¬ 
er ones, while some, where the bending has been greatest, 
have the position of their beds inverted, as before shown in 
Fig. 73, p. 87. 
The strike of the parallel ridges, after continuing in a 
straight line for many hundred miles, is then found to vary 
for a more limited distance as much as 30°, the folds wheel¬ 
ing round together in the new direction and continuing to 
be parallel, as if they had all obeyed the same movement. 
The date of the movements by which the great flexures 
were brought about must, of course, be subsequent to the 
formation of the uppermost part of the coal or the newest 
of the bent rocks, but the disturbance must have ceased 
before the Triassic strata were deposited on the denuded 
edges of the folded beds. 
The manner in which the numerous parallel folds, all si¬ 
multaneously formed, assume a new direction common to the 
whole of them, and sometimes varying at an angle of 30° 
from the normal strike of the chain, shows what deviation 
from an otherwise uniform strike of the beds may be experi¬ 
enced when the geographical area through which they are 
traced is on so vast a scale. 
The disturbances in the case here adverted to occurred be¬ 
tween the Carboniferous period and that of the Trias, and 
this interval is so vast that they may have occupied a great 
lapse of time, during which their parallelism was always 
preserved. But, as a rule, wherever after a long geological 
interval the recurrence of lateral movements gives rise to a 
new set of folds, the strike of these last is diferent. Thus, 
