EFFECTS OF DEPOSITION 295 



thousands of feet below the level at which the waters which excavated 

 them were of old discharged into the sea or into the greater rivers. 



The hypothesis of isostatic movement may serve in part, at least, to 

 explain the often wall-like character of the mountains that face these 

 valleys. As has already been noted, this aspect of these vales is so strik- 

 ing that it demands an explanation. If we assume a continued settling 

 of the floors of the valleys, accompanied by an upward movement of the 

 neighboring ridges, we may well imagine that we would have much 

 faulting about the changing axial line. In other words, each of these 

 valleys would be the seats of more or less considerable step faults, the 

 whole forming a complicated " graben " like basin. There are not lack- 

 ing instances of a like structure in basins which have been deeply filled 

 with sediments. Thus on the eastern coast of the United States we have 

 in the Richmond basin what seems a similar instance of a valley step 

 faulted under a heavy burden.* The Narragansett basin, in Rhode 

 Island and Massachusetts, is a similar instance of a basin that appears 

 to have been downborne by the weight of sediments imposed upon its 

 floor. It is, indeed, likely that this downbending of loaded troughs is 

 a common feature. 



It is to be noted that in the filled basins of the James river and the 

 Narragansett districts of the Atlantic coast, the former of Triassic and 

 the latter of Carboniferous time, we have the same abundant devel- 

 opments of arkose deposits that we find in the broad valleys of the 

 Cordilleras. These facts suggest the hypothesis that the process of occlu- 

 sion of valleys in the manner indicated in the troughs we are consider- 

 ing may be sufficiently common to be regarded as of general importance. 

 The fact that in the eastern basins there are occasional coalbeds in certain 

 parts of the section may mean no more than the temporary occurrence 

 of humid epochs, such as that which has just passed away in the western 

 part of the country. 



It is evident that in case it should be found that deposits of the broad 

 valleys have in a way determined the development of the orogenic move- 

 ments of the region in which they lie, very interesting light would thereby 

 be thrown on the history of the mountain-building in the Cordilleran 

 and other regions of like conditions. At present, however, the value of 

 the hypothesis is not proved, so that it does not merit further discussion. 



There is a peculiar feature in the recent volcanic activity of the Rocky 

 mountains of which we may possibly find an explanation through the 

 action of the valley deposits. This is the occurrence of eruptions at 

 various points at a time when the sites of these outbreaks were remote 



*See Ann. Kept. Director U. S. Geol. Survey, 1897-'98, p. 401. 

 XLII— Bull. Geol. Soc. Am., Vol. 12, 1900 



