


176 STRUCTURAL AND FIELD GEOLOGY 
‘ : 
It is impossible to believe that the folded strata seen in a mountain range 
could have been so sharply curved and plicated without fracture, unless 
as the result of powerful pressure slowly applied. As regards dislocations, 
there is no evidence to show that great rock-displacements have been 
more rapidly effected than conspicuous rock-folds. We need not go so 
far, however, as to infer that all faults have been slow creeps. Some 
dislocations may have been more or less rapidly effected. That crustal 
deformation, as a rule, is really a protracted process, is strongly suggested 
by the fact that folds and faults have come into existence in certain 
regions without disturbing their drainage systems. The Colorado Plateau, 
for example, has been split across by well-marked normal faults, some 
of which have a downthrow of several thousand feet, and can be followed 
for hundreds of miles. The same region also shows some notable 
folds and flexures, both faults and flexures being of relatively recent 
geological age. Yet none of these crustal deformations has disturbed 
the course of the River Colorado, which was certainly in existence long 
before they had been effected. It is obvious, therefore, that flexuring 
and faulting must have taken place so gradually, that the river was able 
to saw its way across the inequalities as fast as these appeared. Similar 
evidence to the same effect is supplied by the river-valleys of the 
Himalayas. It is well known that the sub-Himalayan ranges are com- 
posed of materials derived by existing rivers from the central ranges of 
the great chain. The materials referred to form massive accumulations 
which have been disturbed and upheaved, the axes of the flexures crossing 
the river-valleys more or less directly. Here, then, it is evident that 
“the rivers are older than the hills they traverse, and that the gorges 
have been gradually cut through the hills as they were slowly upheaved.” 
Yet another example may be cited from our own Continent. Deep 
borings have shown that the Pleistocene deposits in the valley of the 
Rhine in Hesse occupy a profound hollow, surrounded on all sides by 
older rocks, the bottom of the basin being 270 feet deeper than the 
lowest part of its rim at Bingen. These deposits, however, are not 
lacustrine, but fluviatile. Hence we must infer that fluviatile deposition 
has kept pace with the crustal movement. As the bottom of the Rhine 
valley has slowly subsided, the river has flowed on without interruption, 
continuously filling up the gradually deepening basin with its sediment. 
