1866.] 11 • [Shaler. 



■without tlie assumption of any thing more than the trifling amount of 

 igneous fluidity involved in the hypotheses we have just discussed. 



Without any particular examination of the facts, it seems to have 

 been assumed by most geologists that all the phenomena of corruga- 

 tion, whether exhibited in mountain ranges, or in continents, are to 

 be regarded as effects of one and the same cause, differing only in 

 magnitude. It is manifest that it is a matter of first importance in 

 seeking an explanation of the origin of these phenomena, to deter- 

 mine whether this assumed identity of cause is true or no. If it be 

 the tact that continental elevations and mountain elevations are but 

 degrees of effect of the same cause, then there should be no other 

 differences in the phenomena than those of magnitude, or of features 

 dependent directly upon the magnitude of the areas involved in the 

 disturbance ; furthermore, there should be something like a series, at 

 one extremity of which could be placed the greatest relief of conti- 

 nental fold and oceanic depression, and passing gradually to the most 

 inconsiderable flexures. It requires no very careful examination to 

 bring the observer to the conviction that those essential features do 

 not exist. The phenomena observable in the two actions are not cog- 

 nate. There can hardly be said to be any thing like a series or 

 gradation connecting the whole assemblage of phenomena, and the 

 inference seems strong that the cause is not the same in the two cases. 

 We find, for instance, in continental folds, broad curves of the surface, 

 which narrow without exception towards the south, and which exhibit 

 in no part of their structure the evidences of powerful lateral thrust, 

 which are the most conspicuous phenomena of mountain chains. In 

 these latter, however, we perceive evidences of linear disruption of 

 the crust, showing intense, but localized energy, with no tendency to 

 increase of magnitude in any one direction. In the continents we 

 behold curves of thousands of miles in diameter, showing an equal 

 force acting throughout, in the mountain very powerful forces acting 

 along one line, and inoperative a few tens of miles away. There 

 seems nothing in common in the phenomena except that both are 

 folds of the earth's surface. The great breadth, and comparatively 

 gentle curves, characterizing the continental folds, show that a great 

 thickness of material is involved in the movement; their gradual 

 development in successive geological periods, together with what we 

 know concerning the loss of heat from the interior of the earth ren- 

 ders it eminently probable that they arise from the accommodation of a 

 hardened outer crust to a diminished nucleus. All the fluidity re- 

 quired in this view of the effect of the contraction of the mass upon 

 the contour of the crust, is given by the hypothesis which claims that 

 solidification began at the centre, and that all that remains in any 

 sense liquid, is a very small portion comparatively near the surface. 



