542 W. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



here sketched; but our acquaintance is limited to a geological moment 

 known as ^^the present/' and our knowledge of even the present condition 

 of many coasts is not yet intimate, for it has not been gained by a scru- 

 tinizing inquiry during which all problems of coastal development were 

 consciously held in mind. It is therefore natural enough that some of 

 the stages of coastal development above outlined should not yet have 

 found their counterparts in nature. The best that we can now do is to 

 discover counterparts for as many stages as possible ; we may then, accord- 

 ing to the success with which some of the deduced stages match the facts, 

 judge of the correctness of the other stages and thus of the whole sequence 

 of changes involved in our theoretical scheme. Coasts of recent or of 

 long-continued submergence will first be considered ; coasts of less recent 

 submergence will next be examined. 



REEFS ON COASTS OF RECENT AND CONTINUED SUBMERGENCE 



Reefs are, as above stated, abundantly developed on coasts of submer- 

 gence in which the embayments are not yet filled with deltas. The lower- 

 ing of the Glacial ocean may have contributed to this result by enabling 

 the streams to wash away the detritus which had previously accumulated, 

 but, as has been shown in several previous paragraphs, the changes of 

 ocean level thus accounted for do not suffice to explain nearly all the ero- 

 sion and submergence that many reef-fronted coasts have experienced. 

 Long-continued, intermittent subsidence, varying in place, amount, time, 

 and rate, and not infrequently alternating with elevation, is demanded 

 by the multitude of varied facts. This point need not be dwelt on longer. 



SMOTHERED REEFS ON COASTS OF LESS RECENT SUBMERGENCE 



The mountainous and embayed coast of southern N"ew Gruinea is fronted 

 by barrier reefs for much of its length, but in a large reentrant near its 

 middle the Fly River — named after the British exploring vessel on which 

 Jukes was geologist — has formed an extensive delta prolonged in muddy 

 shoals ; here reefs are now absent, but it is highly probable that they were 

 present along parts of this coastal reentrant before the delta gained its 

 present size. The great deltas of the Irrawaddy in Burma and of the Me- 

 kong in Cochin China are built forward from coasts of fairly strong relief, 

 that have been embayed and morcellated by submergence ; reefs are Want- 

 ing on the delta deposits, but they occur in patches on the adjoining 

 coasts. Many smaller examples of this kind are known in the lagoon of 

 the Great Barrier reef of Australia, where advancing deltas liave appar- 

 ently smothered previously formed fringing reefs. Certain stretches of 



