740 ABSTRACTS OF PAPERS 



this coast within the last tew thousand years. The phenomena which seem 

 to indicate recent subsidence appear to fall into three groups: (1) Fictitious 

 appearances of subsidence which are produced bj' wave action on a retro- 

 grading shoreline without any change in the level of land or sea ; to this 

 group belong many instances of submerged stumps, peat exposed at low water 

 on the seaward side of barrier beaches, erect trees recently killed by the in- 

 vasion of salt water, etc. (2) Phenomena produced by a local rise in the 

 high tide surface, due to a local change in the form of the shoreline, unac- 

 companied by any general change in the relative level of land and sea ; in this 

 group may be found examples of practically all phenomena ordinarily attrib- 

 uted to a recent subsidence of the land. (3) Phenomena produced by an 

 actual subsidence of the land or rise of the sealevel which occurred some 

 thousands of years ago ; in this group belong -many of the deeply buried peat 

 deposits and submerged stumps. The evidence of coastal stability consists of 

 ( 1 ) the form and position of successive beach ridges, the oldest of which were 

 built by the waves thousands of years ago, yet later than the deeply buried 

 peat deposits; (2) the position of abandoned marine cliffs on which the waves 

 can not have worked in recent time, and (3) the absence of a fringe of dead 

 trees on those portions of the coast which are exposed neither to direct wave 

 attack nor to local fluctuations of the high tide surface. It is concluded, with 

 reference to the Atlantic coast, that the land can not have subsided as much 

 as a foot within the last century ; that there can have been no long continued 

 progressive subsidence at so high a rate as one foot per century, within the 

 last few thousand years, and that no evidence thus far available can be re- 

 garded as satisfactory proof of any degree of recent subsidence, either spas- 

 modic or progressive. 



Discussion 



Dr. C. A. Davis : The physiographic evidence to be gathered on the exposed 

 shores is not such as to show slight changes of level as the range of the tides 

 is so great ; hence recent subsidence can neither be proved nor disproved by it. 

 While we have the possibility of a general elevation of the tidal surface be- 

 cause of changes in the shores of the Atlantic, there is no evidence that this 

 has occurred or that such elevation is going on either in tidal records or 

 elsewhere. The evidence of the salt marshes that subsidence is going on, since 

 it is to be presented in a paper to be read at this meeting, will not be men- 

 tioned now, but it may be said that at Nantasket Beach there is a small area 

 of salt marsh which shows recent change of level, in that if there had been 

 stability for as much as 1.000 years it could not exist as a salt marsh, be- 

 cause the plant remains would have built up above the reach of the tides. 

 Four or five miles southwest of Nantasket is an area of salt marsh with 

 stumps of trees in place more than 2 feet below the level of ordinary high 

 tides, and partly buried in salt marsh peat, which must have been formed 

 by the gradual change of the relation of the high tide to the bottom. No 

 sudden change in the relation would account for this accumulation, and the 

 fhange has not yet ceased. The tidal waters find their way to this area by 

 tortuous channels, and any change of tidal level on the marsh must have 

 been accompanied by greater ones at the beach, as the tidal wave has more 

 direct access to it. 



