AGE OF DISLOCATIONS. 41 



the Jura. In most cases the crests of the anticlines have been widely 

 opened by erosive processes, and in some rare instances the destruction lias 

 advanced so far that the synclinal element in the foldings has come to lie 

 farther above the neighboring drainage than the existing crests of the 

 upfolds. Notwithstanding this excessive local downwearing which has here 

 and there taken place, the West Appalachians have everywhere, except in 

 their extreme southern part, retained a striking topographical relief. It is 

 indeed easy to see, even in the most ruined part of these great geological 

 edifices, the plan of the structure and the general features of their archi- 

 tecture. It is quite otherwise with the related elevations of the Atlantic 

 coast. As before noted, the East Appalachians have, in their topographical 

 expression, scarcely a semblance of the structure of the West Appalachians. 

 In fact, their lack of relief has to this day hidden from geologists their real 

 importance as orogenic phenomena. 



Between Georgia and the Bay of Fundy none of these mountains have 

 any distinct topographical relief. Here and there the crystalline rock which 

 were formed under their anticlines, or the massive outbreaks of igneous 

 rocks which took place during the folding, remain as considerable hills, or 

 in the case of -the Mount Desert elevations they may attain the height of 

 1,000 feet or more; but in the Narragansett Basin, although the folds cer- 

 tainly have a geological relief of not less than 10,000 feet, the actual differ- 

 ences in altitude from the depth of the present water channels to the highest 

 elevations does not exceed 500 feet. If these mountains of the East 

 Appalachians had been no more worn down than the Alleghenies, they 

 would afford the most majestic elevations in the eastern part of the continent, 

 instead of having no distinct value in respect of topographical relief. 



It might at first be supposed that the age of these eastern reliefs is 

 greater than that of the western dislocations; the evidence, however, points 

 to the conclusion that, while some part of the dislocations may be due to 

 stresses which were of Cambrian or Silurian age, the greater of these 

 accidents date from post-Carboniferous times, and are probably to be 

 assigned to the age of the Trias or the Jura. The disturbances which have 

 contorted the Cretaceous or Tertiary rocks of Marthas Vineyard clearly 

 indicate that the orogenic forces have acted along the Atlantic coast with 

 much energy down to very modern time. To what, then, can we attribute 

 the very great differences in the relief of these two mountain-built districts? 



