228 DALY. 



Central-island bays of this origin are probably shorter and less con- 

 spicuous than those due to the drowning of stream -erosion valleys, 

 but are none the less worthy of attention by the student of shore 

 topography. 



Again, as noted on page 162, a complete analysis of the bay problem 

 must take account of the possibility of a Recent shift of sea-level, 

 owing to causes other than the melting of glaciers. Post-Glacial 

 uplift of the sea floor, over extensive areas, has been proved. If it 

 has not been wholly compensated by sinking of the ocean bottom 

 elsewhere, a general rise of sea-level occurred in post-Glacial time. 

 Such a positive movement would tend to drown the valley-in-valley 

 "edges," the Pleistocene shore cliffs, and allied topographic features. 

 A post-Glacial rise of a few meters is quite possible as the result of 

 diastrophic processes. 



On the other hand, the level of the ocean to-day cannot be many 

 meters from its position in the Pliocene period. (See page 198.) 

 That conclusion follows from the facts expressed in the charts of the 

 continental shelves. Whatever may be the shapes of the rocky 

 terranes beneath the shelves, the surface of each shelf has surely been 

 smoothed and greatly widened by waves and currents. Each repre- 

 sents an embankment growing, like a delta, into deep water. Normal 

 storm waves and ocean currents effectively transport bottom mud if 

 the depth of water is 75 m. to 40 m. or less. Depths of 75 m. to 40 m. 

 prevail in the outer half of each of the wider shelves throughout the 

 ocean. The building out of the great embankments to their actual 

 widths demands all the time from at least the mid-Pliocene to the 

 present day. The continental shelves seem, therefore, to indicate 

 nearly the same position for sea-level during the later Tertiary as for 

 post-Glacial time. The Pleistocene shifts of level represent a com- 

 paratively brief interlude, and there is no evidence that the major 

 shifts of that period were essentially caused by any other process than 

 glaciation and deglaciation. 



Finally, the absolute proof of bay-making by Recent subsidence 

 would not establish general subsidence for all areas characterized by 

 barrier reefs or atolls, nor would it invalidate the Glacial-control 

 theory. Recent warping or faulting of the earth's crust, in moderate 

 amount, is an obvious fact in the Fijis, in the Tonga archipelago, in 

 New Caledonia, in Oahu of the Hawaiian group, and in some other 

 oceanic localities. The elevated strand-lines of the uplifted parts 

 have correlatives in the drowned valleys of the sunken parts. As 

 above noted, the Pleistocene platforms have been simultaneously 



