STRATIGRAPHIOAL AND OROGENIC RELATIONS. 13 



Those on the west, are narrow, relatively long, and consist, with slight 

 exceptions, of simple foldings of the true anticlinal type. Those in the 

 eastern or seaboard district are in general rudely oval in form, their length 

 usually not exceeding twice their width. They are, in fact, broad troughs, 

 the included strata being cast into a number of anticlines and synclines. 

 This peculiar difference of form leads naturally to the supposition that the 

 history of these two groups of depressions has been diverse. An inspection 

 of the deposits verifies this supposition. It has already been stated that 

 the Narragansett Basin was an ancient trough, formed before the Carbon- 

 iferous period, in which, during a process of subsidence, the beds of the 

 Coal Measures were accumulated. The evidence derived from the study 

 of the Richmond, the Connecticut River, the Boston, the Mount Desert, 

 and the Passamaquoddy basins has satisfied me that the troughs are of 

 ancient date, that they were filled to a considerable extent with materials 

 imported from the higher country about them, and that this filling process 

 was associated with progressive local subsidences. 



The foregoing considerations seem to me to warrant the supposition 

 that the East Appalachian basins, or at least the greater part of them, were, 

 in the beginning of their formation, erosion troughs, which became the 

 seats of excessive deposition, and this brought about the lowering of their 

 surfaces in relation to the original bed. In a word, they were downpressed 

 by the weight of the burdens which came into them. At a subsequent 

 stage the mountain-building forces, acting irregularly, compressed these 

 troughs, producing the sets of local disturbances which are exhibited by 

 each field. A possible instance of such local orogenic action in very 

 modern times, as late as the Pliocene, is found in the tilted strata of the 

 Marthas Vineyard district. In this case excessive deposition of a local 

 character has been followed in turn, first, by subsidence, and then by com- 

 pressive action, producing a large measure of folding, in a general way like 

 that which has taken place in the neighboring Narragansett Basin, which 

 lies immediately on the other side of the anticline that forms the eastern 

 boundary of the Narragansett trough. 



It should be said that the hypothesis of the antecedent erosion of the 

 basins which we are discussing has a considerable measure of support from 

 the very diverse orientation of the axes of the East Appalachian troughs. 

 These range from east and west to north and south, a diversity which 



