]4 GEOLOGY OF THE NARRAGANSETT BASIN. 



seems to me to be inconsistent with the supposition that they have been 

 formed through the action of such accurately determinate strains as 

 produced the Alleghenian ridges. The last-named foldings are of the 

 normal mountain type. The axes are parallel for great distances, and 

 where the ridges change their orientation they preserve their parallelism 

 and alter their general course with rather gentle curves. They exhibit no 

 case of such contrast in the axes of the basins as is shown in the adjacent 

 Narragansett and Boston troughs. 



The known facts concerning the effects arising from the accumulation 

 of thick sediments warrant the supposition that wherever this action occurs 

 it is likely to be attended by a subsidence which, though of a local char- 

 acter, may attain an extent proportionate to the influx of the de"bris. 

 Wherever along the coast line long-continued land erosion forms deep 

 valleys, these depressions are likely to be, during a period of subsidence, 

 the seats of extensive deposition. Where the amount of this sediment is 

 sufficient to develop the downcast movement, it may lead to the formation 

 of a trough of great geological depth, though it may at all times be shallow 

 water or even retain the state of a delta area. It therefore does not seem 

 a matter for surprise that the Atlantic coast district should exhibit basins of 

 this nature, for, although this coast of the continent has been subject to 

 many alterations of level, there is abundant evidence to show that from the 

 Cambrian period to the present day the eastern front of the land has been 

 often, indeed we may say prevailingly, somewhere near its present position. 

 There has been ample time for the formation of many great coast erosion 

 troughs and for their filling with sediments to the amount which the 

 hypothesis requires. The final development of anticlines within the troughs 

 in the extensive way in which they appear to have been formed may 

 readily be explained by the existence of the same compressive tensions 

 which have operated in the West Appalachian field, the difference being 

 that in the East Appalachians the form of the troughs somewhat controlled 

 the direction of these anticlines, while in the western portion of the system, 

 a newly emerged part of the continent, they appear to have been guided 

 in their alignment in a much greater measure by the direction of the com- 

 pressive strains, there being no strong topographical features except the 

 old land on the east to determine the trend of the upcurving. 



At the present time, although the Atlantic coast of North America has 



