JAMES DWIGHT DANA. OI! 
contracting and non-contracting (comparatively non-contracting) areas. 
The efficiency of the cause of contraction has appeared to the writer to 
be wider and more evident, as the subject has received closer attention ; 
and the study of it very naturally led to modifications of former views.”’* 
Thus, it will be seen, that although others had before con- 
ceived of the idea of the general effects of contraction, it was to 
Dana the working hypothesis in the construction of a system of 
geology. 
Although later investigations have added new light for the 
interpretation of the details of mountain building and earth 
shaping, a reference to the chief points of the theory, as elabo- 
rated by Dana in 1847 will show how much we are indebted to 
him for a clear exposition of the general principles of the sci- 
ence. In the second article ‘‘On Geological Results of the 
Earth’s Contraction” these principles are stated. In regard to 
the Appalachian Chain, the general structure of which the 
Rogers brothers had already elaborated, the peculiarities were 
by them explained as the result of the propelling force or thrust 
of moving waves of molten material beneath, and the ‘“ disrupt- 
ing tension of the compressed gaseous matter.” Mather had 
previously spoken of the effects of refrigeration of the earth, 
but he found the cause of the features of the Appalachians in 
“a paroxysmal elevation and the action of inertia due to the 
more rapid westward motion of the part of crust lifted up fur- 
ther from the center of revolution.” In the article above 
referred to, Dana wrote, ‘The; principal peculiarity of these 
plications to which we would ask attention, is the following: 
the greater abruptness of the northwestern slope of each fold, 
in connection with the diminution of the undulations to the 
northwestward; and it will be our endeavor to show that this 
peculiarity, and the irregularities which exist are necessary 
results of the action of a force laterally exerted ;” and he pro- 
ceeded to demonstrate how by this power, ‘fa series of folds 
would be produced each with the inclination steepest on the side 
farthest from A (the point of resistance); and moreover, these 
*Am. Jour. Sci. II., Vol. IIL., p. 179, 1847. 
