34 GEOLOGY OF THE NAKKAGANSETT BASIN. 



movement is essentially like the rotation of the lever about the neutral or 

 fulcrum point, which is ordinarily near the shore line. On the doctrine of 

 probability, it is more likely to fall at the contact of land and sea than at 

 any other point in the length of the rotating area. If the land advances 

 from the ancient shore, the natural result is an increase in the amount of 

 erosion and consequently of deposition off the given coast line. This, in 

 turn, as an effect of the loading, tends more sharply to depress the region 

 next the coast, and so, in time, to a return nf the shore toward its original 

 position. If, on the other hand, the sea invades the land, considerably 

 narrowing the field of erosion, the supply of sediments is checked and the 

 element of accumulating weight which makes against the uprising is 

 proportionately lessened, with the resulting tendency of the district to 

 ascend in the next adjustment of the crustal stresses which are involved in 

 continental growth. 



So far as I am aware, the mountainous elevations which have been 

 formed along the Atlantic coast appear to have been the result of stresses 

 which have acted in a somewhat continuous manner from the Cambrian to 

 near the present day. The dislocations seem to have occurred in these 

 basins as early as the first-named time, and in the basin of Marthas Vine' 

 yard they operated perhaps until the first stages of the last Glacial epoch. 

 It is true that the evidence as to the distinct basin-like position in which 

 the rocks of Marthas Vineyard lie is not very clear, for the reason that the 

 eastern wall, if such wall existed, is now below the level of the sea; but 

 the mountain-building nature of the disturbances appears to be unquestion- 

 able, the original folds having had a geological height of several hundred 

 feet, though, owing to the soft nature of the strata, they have now been 

 reduced to near their base-levels. 



It is a notable fact that in these erosion-basin mountains of the Atlantic 

 coast there is a manifest tendency of the streams to return again and again 

 to somewhere near the paths from which they have been displaced by the 

 subsidence of the areas beneath the sea or by the corrugation of the beds 

 which were formed during these periods of depression. Thus in the case 

 of the basins along the coast of Maine, those of Boston, the Connecticut, 

 and the set about the Chesapeake, streams answering to the original agents 

 of erosion now occupy their ancient sites. It is evident that in this 

 particular, as in many other features, the dislocation areas we are consider- 



