102 
MOUNTAINS AND THEIR ORIGIN. 
subsidence of the crust occurred, the consequent 
pressure upon the melted materials beneath must 
displace them and force them upward. While 
the crust continued so thin that these results 
could go on without very violent dislocations, — 
the materials within easily finding an outlet, if 
displaced, or merely lifting the surface without 
breaking through it, — the effect would be mode- 
rate elevations divided by corresponding depres- 
sions. We have seen this kind of action, during 
the earlier geological epochs, in the upheaval of 
the low hills in the United States, leading to the 
formation of the coal-basins. 
On our return to the study of the American 
continent, we shall find in the Alleghany chain, 
occurring at a later period, between the Carbon- 
iferous and Triassic epochs, a good illustration 
of the same kind of phenomena, though the ac- 
tion of the Plutonic agents was then much more 
powerful, owing to the greater thickness of the 
crust and the consequent increase of resistance. 
The folds forced upward in this chain by the sub- 
sidence of the surface are higher than any pre- 
ceding elevations ; but they are nevertheless a 
succession of parallel folds divided by correspond- 
ing depressions, nor does it seem that the dis- 
placement of the materials within the crust was 
so violent as to fracture it extensively. 
Even so late as the formation of the Jura 
