722 



SCIENCE 



[N. S. Vol. XXXII. No. 829* 



three quarters of a foot nearer the mean level 

 of the sea above which it was placed than it 

 was when first put in position. 



More or less opposed to the theory of sub- 

 sidence are certain suggestions which have 

 been advanced to account for the foregoing 

 lines of evidence without invoking a wide- 

 spread subsidence of the land. Among these 

 suggestions we may note the following : beach 

 sands and sand dunes encroaching on a marsh 

 will so weight it down as to cause local sub- 

 sidence; draining marshes may cause a set- 

 tling of from one to several feet; fresh 

 marshes and forests may occupy depressions 

 separated from the ocean by a barrier beach, 

 under such conditions that a breach in the 

 barrier would admit the sea to kill the trees 

 and cover the fresh-water peat with salt veg- 

 etation; changes in direction or velocity of 

 ocean currents may cause local changes in 

 mean sea level; variations in tidal compo- 

 nents having from sis-hour to nineteen-year 

 periods may cause long period oscillations of 

 sea level; abnormal variations of atmospheric 

 pressure, recurring in three- and eight-year 

 periods, cause periodic variations in the level 

 of the sea. 



The writer would call attention to a factor 

 which produces fictitious appearances of 

 coastal subsidence, and which he believes to 

 have a higher degree of importance than any 

 of those mentioned above. As a tidal wave 

 approaches an irregular coast it is materially 

 modified in shape and in height. If a surface 

 could be constructed to pass through every 

 point reached by the crest of the tidal wave, it 

 would be found to have marked undulations of 

 considerable complexity. The surface would 

 rise well above mean sea level in bays which 

 are widely open at their mouths and converge 

 toward their heads; but would descend 

 abruptly toward mean sea level where a nar- 

 row inlet connected the ocean with a broad, 

 land-locked bay or lagoon. Within such an 

 enclosed bay this " high-tide surface " might 

 be a number of feet lower than that portion 

 of the surface immediately outside of the en- 

 closing arms of land. 



The irregular high-tide surface is very un- 



stable, and will undergo modifications as; 

 waves and currents erode islands, build bars, 

 silt up or scour out channels, break through 

 barrier beaches, or otherwise modify the 

 shoreline and adjacent shallow water areas. 

 Where waves break through a bar enclosing a 

 bay which was formerly connected with the 

 ocean by a narrow inlet, the high-tide sur- 

 face within the bay may instantly be raised 

 several feet, since the broader opening per- 

 mits the rising waters to enter freely and 

 so give tides within the bay as high as those 

 in the adjacent ocean. A more gradual en- 

 largement of the inlet would cause a gradual 

 elevation of that portion of the high-tide sur- 

 face within the bay; whereas a growing bar 

 might cause a decrease in the height of the 

 same surface. If the size of the inlet remains 

 constant, then silting up of the bay, the en- 

 croachment of tidal marshes, or the reclaim- 

 ing of part of the bay surface by artificial 

 filling or by the construction of dykes, will 

 cause a raising of the high tide surface 

 within the remaining areas of the bay ; for the 

 water entering through the narrow inlet, hav- 

 ing less area to spread over, will accumulate 

 to a greater depth than formerly. 



ISTow it is the irregular and changeable high- 

 tide surface, rather than the mean sea level, 

 which is most important in discussions of 

 coastal subsidence. Tidal marshes build up to 

 the level of the high-tide surface; and owing 

 to the inequalities of this surface, marsh level 

 may vary a number of feet in closely adjacent 

 areas. A breach in an enclosing bar, a widen- 

 ing of a tidal inlet, artificial encroachments 

 on the bay area, or other shoreline changes, 

 may cause the high-tide surface to rise locally. 

 Salt water will then invade the adjacent 

 forested slopes and kill the trees; salt marsh 

 deposits will build up to the new high level, 

 encroaching on the upland areas, surround- 

 ing and later burying the stumps of the killed 

 trees, and covering the fresh-water peat formed 

 along the margins of the fresh marsh and 

 along the rivers emptying into the bay, with 

 salt-water peat; ancient landmarks will be 

 submerged, Indian shell heaps will now be 

 found below the high-tide level, old dykes on 



