226 DALY. 



young valleys ; a relatively broad bay may not be a drowned " mature " 

 valley, if by that term is meant a valley of great absolute age. The 

 point specially worthy of note is that one can not, in these cases, 

 safely locate the original bottom of each drowned valley at the inter- 

 section of the visible valley slopes, simply prolonged without essential 

 change of angle to the horizontal plane. 



When the sea-level of the coral seas fell, during the first Glacial 

 climax, the floors of the broader stream valleys were trenched by the 

 now revived streams. An "edge," "shoulder," or break of slope was 

 then formed where the Tertiary floor met the top of the incised 

 Pleistocene trench. Assuming no crustal movement, this "edge" 

 was a few meters higher than the present sea-level, and at first sight 

 it seems necessary to expect the break of slope to be visible to-day. 

 The failure to find such valley-in-valley remnants in the present topog- 

 raphy of certain Pacific islands has led Davis to doubt the Glacial- 

 control theory. 



Yet, if the inner valley were essentially completed during the early, 

 Kansan stage of glaciation (probably the time of maximum ice in 

 North America at least), it is unlikely that the "edge" would still be 

 generally, if at all, preserved. Post-Kansan time has been long enough 

 for the mature dissection of the Kansan drift. The rock-material 

 forming the "edges" of inner valleys must have been somewhat weak- 

 ened by weathering; otherwise no "edge" would have been devel- 

 oped, since the widening of such valleys depends on the preliminary 

 weathering of the rocks in the valley sides. Post-Kansan time, fav- 

 ored by the rapid rock-decay and heavy rains characteristic of the 

 tropics, seems long enough to have largely or quite obliterated such 

 minute features as these valley-in-valley " edges." 



Upstream, each revived river or creek must have speedily cut a 

 distinct, narrow gorge in the floor of the Tertiary valley, just as the 

 rivers of England and many other countries have cut young gorges 

 during the Pleistocene. Those gorges should still exist, but, in general, 

 they must be largely filled with post-Glacial alluvium and be thus 

 invisible at the present surface. 



That the Glacial period was long enough for the excavation of inner 

 valleys 50 or 60 m. deep, is not an extravagant assumption. If the 

 Antarctic ice-cap, the last surviving one of great size, was also the 

 first to form, the axial parts of the valley floors of the islands, except 

 the lower stretches now submerged, have suft'ered subaerial erosion 

 during a period longer than all Kansan and later time. 



Reviewing the criterion, it appears, first, that some bays of central 



