62 
DUTTON. 
disturbance or after it has reached a sufficient amount, and 
cease with it. These folds, however, have been subject since 
their first formation to great erosion, which is also a disturb¬ 
ance of isostasy, and thus the original plication may have 
been increased or modified thereby. 
The theory may also be applied in a most satisfactory 
manner to the explanation of subordinate features associated 
with plication. 
(5.) One of the features of plication which has attracted 
great attention and occasioned great perplexity to geologists 
is the so-called fan-structure. This is very striking in the 
Alps, and has its counterpart in the inclined folds of the 
Appalachians of Pennsylvania, where the northwestern 
branches of the anticlines are steeper than the southeastern 
branches. If we assume that as the rocks lie deeper in the 
earth they are softened somewhat by the increasing heat, it 
follows that in the flow of the mass the movement would be 
easier and more rapid below than above. Thus a horizontal 
force arising from this differential movement acts upon the 
inverted arches of the synclines and carries their lower ver¬ 
tices forward in the direction of motion. 
Thus the general theory here proposed gives an explana¬ 
tion of the origin of plications. It gives us a force acting in 
the direction required, in the manner required, at the times 
and places required, and one which has the intensity and 
amount required and no more. The contractional theory 
gives us a force having neither direction nor determinate 
mode of action, nor definite epoch of action. It gives us a 
force acting with a far greater intensity than we require, but 
with far less quantity. To provide a place for its action it 
must have recourse to an arbitrary postulate assuming for 
no independent reason the existence of areas of weakness in 
a supposed crust which would have no raison d’etre except 
that they are necessary for the salvation of the hypothesis. 
Before closing this discussion it will be necessary to advert 
to another one of the great problems of physical geology, viz., 
the cause of general elevations and subsidences. I do so, 
