1913] JOHNSON—COASTAL SUBSIDENCE 463 
cause the appearance of slow changes of level. Such changes are 
more common, but less striking, than those which are more sudden. 
6) The total apparent subsidence produced in this manner may 
considerably exceed one-half of the tidal range in the adjacent 
sea. A considerable thickness of high tide salt peat may thus be 
produced without coastal subsidence. 
c) Half-tide level does not necessarily remain the same after 
the change in high tide level. 
d) The application of the principle here set forth is not restricted 
to bays of the type shown in fig. 6. Vast lagoons parallel to the 
coast, such as those of Long Island and New Jersey; open bays 
whose mouths are being widened by wave erosion; salt marshes 
traversed by meandering tidal channels; the intricate network of 
passes between the low and changeable islands of South Carolina 
and Georgia, or parts of the Holland coast; all these present 
favorable conditions for local changes of tide level consequent — 
upon changes in the width, length, or depth of the tidal channels. 
e) Many bays now open to the sea were once doubtless more 
or less nearly closed by barrier beaches. This is especially true 
along glaciated shores where the waves and currents have effected 
in post-glacial time, and are still effecting, comparatively rapid 
changes in the form of the shore line. 
f) Appearances of subsidence predominate over those of eleva- 
tion because marsh deposits tend to sink to the new level when 
high tide level is lowered; because the immediate destruction of 
fresh water vegetation by salt water when the high tide limit is 
raised is more striking than the slow recovery of marine area by 
fresh vegetation when the high tide level is lowered; and because 
in the cycle of shore line development retrograding exceeds (fi 
grading, and retrograding tends to carry higher tide levels into low 
lands where apparent changes of level are most easily reco; a. 
In order to determine how far conditions along the Atlantic 
coast were favorable to such future changes in high tide level se 
would produce apparent coastal subsidence, we ge poe 
comparison of the height of the same high tide in partia a : = 
bays, in lagoons, and in tidal creeks of salt marshes on a 
hand, and in the ocean on the other hand, at @ large num 
Bike, 3 ae Se he 
