34- JOURNAIv OF THE 



the marg-iii of the ocean that causes the migration. If 

 this be in the nature of an uplift the sea will recede, 

 the shore line successively occupying positions further 

 and further out upon what was formerly sea-bottom. 

 If on the other hand a subsidence of the land surface 

 takes place, the sea will transgress the land and the 

 shore line will successively occupy higher and higher 

 parts of the land slope, that is, further and further 

 inward from its former position. 



Slight changes in the position of a coast line may 

 take place in other ways than by bodily movements of 

 the land mass, namely by the deposition of material on 

 the margin of the sea-bottom, thus causing the shore 

 line to recede from the land and by removal of mate- 

 rial from the shore, thus causing the sea to transg'ress 

 the laud. These causes are however productive only 

 of comparatively small migrations when acting alone. 

 Evidently we must look upon oscillations of the land 

 mass as the chief cause of change in the position of 

 shore lines. 



In order to get at the history of such a shore line as 

 that of the Atlantic of North America, we must know 

 how to read the evidence of its former presence in 

 places other than that now occupied by it. What are 

 these evidences ? Probably the most direct, as well as 

 the most exact, evidence is furnished by the actual and 

 characteristic marks left in the form of raised beaches 

 or bench marks. Another evidence is furnished by the 

 position and character of sedimentary deposits, — 

 though the absence of evidence of either kind does not 

 necessarily mean that shore conditions were never pres- 

 ent there. While the presence ,of beach marks fur- 

 nishes more exact evidence of shore line position in cer- 

 tain cases, the application is not of so wide extent as 



