18G8.] 129 [Shaler. 



few tens of miles in extent, is to be found in rapid process of change 

 of level adjacent to immovable lines of shore ; on the contrary, the 

 movements, while tbey may vary iji force, and fade away in any 

 direction, do so with a regularity wliieh shows that the force is very 

 widespread and uniform in its action. Another important feature is, 

 that an upward movement on one portion of a coast may gradually 

 diminish and \ydss into a subsidence on the same coast line. Looking 

 still further, we perceive some very peculiar features in the distribu- 

 tion of the changes of level which are still going forward, or which 

 have taken place since the close of the glacial period. All those 

 regions which exhibit distinct evidence of the former existence of the 

 ice enveloiDC, show also unquestionable proofs of elevatory action, 

 since the passing away of the glacial condition. A still more intimate 

 connection between glaciation and change of level is exhibited in the 

 increase in the elevation of ancient sea margins, as we go from the 

 regions where the ice had its southern limits, towards those points 

 where there is any evidence to convince us that it attained the great- 

 est thickness. Although our information wants the jirecision which 

 can make accurate comparison possible, there is no doubt that the 

 increase in thickness of ice was attended by something like a propor- 

 tionate augmentation in the submergence of the land. The brief 

 period during which these changes of level have been subjected to 

 careful investigation, has shown no case where the same point has 

 been alternately under the influence of elevatory and depressive 

 actions, but fi*om the study of the record of such movements since the 

 glacial period, we have abundant reason to believe that such alterna- 

 tions ai-e very frequent, and that it is the exception, rather than the 

 rule, that the same point on the shore has continued under the influ- 

 ence of either elevatory or depressing forces during the whole of the 

 present period. 



Upward and downward movements, with long intervals of rest, 

 during which beaches were formed, and precipices excavated by cen- 

 turies of incessant wave action, are shown along all the coasts lying 

 within the glacial zone. 



The great length of the shores on which we find evidence of the 

 rising or sinking of the land, and the prevailing uniformity of move- 

 ment over great areas, and in great lengths of time, require us to 

 suppose that a great thickness of the earth's crust partakes of the 

 motion, and that the forces producing the change are constantly 

 in action, and not of that instable and transitory character which 

 belongs to ordinary volcanic agencies. At the same time, we have 



PROCEEDINGS B. S. X. H.— VOL XII. 9 NOVEjrBER, 18(38. 



