202 N. S. SHALER — PLEISTOCENE DISTORTIONS. 



of a basin might well be below the ocean-level. It should also be noted 

 that the gneissic rocks along the mainland from New Bedford eastward 

 decline in steep slopes beneath the sea in the manner which we should 

 expect if they represented the margin of a synclinal trough. 



In conclusion, it may be remarked that the orogenic movements of 

 southeastern Massachusetts evidently occurred shortly after a very ex- 

 tensive importation of detritus into the district took place. The exposures 

 at Gay Head and elsewhere show that a thick section was deposited with 

 great rapidity. At no other point on the Atlantic coast of this continent 

 do I know of a case where such a great mass of sediments has been 

 accumulated in a similarly brief period. It is therefore interesting to 

 see that this change of burden was followed by a period of dislocation. 

 The facts, in a word, seem to confirm the hypothesis that transfers of 

 sediments tend in some way to excite mountain-building actions. 



No Evidence of massive Elevation. 



It is important to note the fact that notwithstanding the large amount 

 of crumpling which has taken place in the Marthas Vineyard section, the 

 beds have not been subjected to much massive elevation. They were 

 evidently formed in positions very near the level of the sea. They are 

 still in a general way at about the same position. Similar data concern- 

 ing other formations indicate that this portion of the continent has at 

 several stages in its development been at about its present height above 

 the ocean. Some of the lower Cambrian beds of Attleboro, Massachusetts, 

 contain abundant pebbly waste, which apparently was deposited in rather 

 shallow water. The conglomerates of the Roxbury series, which are pre- 

 sumably of Potsdam age, are clearly coastal deposits. The Carboniferous 

 series of the Narragansett basin was also formed near the sealevel. The 

 intermediate periods doubtless saw great changes in the altitude of the 

 surface in this southeastern district of New England, but the facts seem 

 to show a tendency of the land to return to the level which it has at 

 present. 



