ol4 Notices of Memoirs — iV. S. Shaler 



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

 tinental 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 

 cognate. There can hardly be said to be anything 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 sur- 

 face, 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, characterising 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. 



While the contour of the continental folds, as exhibited both in 

 land surface and sea floors, evinces the gradual operation of the 

 general contraction of the earth on a crust of great thickness, we 

 have in mountain chains another effect of contraction, which cannot, 

 from the evidence, be properly referred to the shrinking of the whole 

 mass. It is evident that if the continental folds are compensative 

 v/rinkles formed in the adaptation of a crust to a diminished nucleus, 

 the mountain chains can not be of the same nature ; it is not to be 

 believed that a crust would bend from the action of the same force 

 into the broad, low curves of the continents, and into the sharp 

 defined and narrow fractures of a mountain range. 



Accepting, as established, the fact that mountain chains are the 

 result of lateral pressure, and indirectly of contraction from loss of 

 heat, and denying that they are the result of the accommodation of 

 the crust to the nucleus, it is at once manifest that we must seek 

 their origin in the changes going on within the crust itself, and in 

 no way connected with the regions below. And within that crust 

 we can find forces operating to produce contraction quite sufficient 

 to account for all the facts. 



