Mr J. D. Dana on the Origin of Continents. 241 



yet ceased action.* Here, then, are the natural effects of 

 proximity to a region of contraction — the Pacific — in which 

 the remains of igneous action everywhere abound. 



It has been well established that the Appalachian folds or 

 plications were made since the coal period, for the coal beds 

 are enclosed in the folds ; \ and the rising of the Rocky chain 

 was also subsequent to that era. The effect of contraction 

 in producing these elevations, was therefore comparatively 

 little felt in the very earliest ages, when the surface of the 

 depressed (or igneous) portion was itself somewhat yielding, 

 but subsequently, when it had become stiffened to a con- 

 siderable depth by cooling. There appears hence to be a per- 

 fect harmony between the results and the causes adduced. 



If these conclusions are correct, we must give up the po- 

 pular idea (at least as a general theory) of the elevation of 

 mountains by a force below causing at the time an irruption 

 of igneous matter ; for the irruption is in general an effect 

 of a very different action, as has been urged by Prevost. This 

 may be as true of the Urals, as of the Rocky Mountains and 

 Andes. 



Even the trap-dykes of New England and New Jersey, 

 whose general course corresponds with that of the Appala- 

 chians, may be a result of the contraction in progress subse- 

 quent to the coal era. The dip of the new red sandstone 

 accompanying them is probably another effect. The Ozark 

 Mountains, forming a line parallel with the Appalachians, 

 beyond the Mississippi, may be referred to the same system 

 of changes. 



The economical advantages belonging to the features of 

 North America that have thus originated, are most remark- 

 able, and this view of their origin gives them increased in- 

 terest. The Silurian rocks indicate that before the coal 

 period the region was comparatively level, and lay mostly 

 beneath the sea. As it emerged it was still dripping with 



* Granites may have been the earlier products ; but the existing volcanic 

 mountains have basalts and trachytes for their surface rocks. 



t W. B. and U. D. Kogers, Trans. Assoc. Amer. Geol. and Nat., 1840-1842, 

 p. 522. 



