Davis. — Signi/icaiif Ftafures of E(< f-liordfird Coasts. 17 



to the great continental shelves in origin, though not in size. The con- 

 tinuity oi the " platform " where a surface barrier reef is wanting, especi- 

 ally on the leeward side of its circuit, may be readily explained as resulting 

 from the continued action of reef-building and lagoon-flooring processes, 

 during subsidences of varying rates and pauses of varying duration, under 

 the influence of prevailing winds. The location of a reef a short distance 

 back of a platform-margin is perfectly consistent with the production of the 

 platform as a mature reef plain, afterwards submerged, and by no means 

 demands that the platform shall have been made iDy processes in w^hich 

 reef-growth had no part. The discussion of this point by observers on reef- 

 encircled islands is much to be desired. 



Partly Emerged Coasts op Submergence. 



A peculiar class of coasts of emergence, mentioned above as needing 

 special consideration, includes mountainous land-borders that have partly 

 emerged shortly after a greater submergence at too rapid a rate for the 

 upgrow'th of barrier reefs. They are characterized by irregular shore-lines 

 with manv salients and re-entrants, on which a comparatively thin cover 

 of marine deposits accumulated during their brief submergence hardly con- 

 ceals the hill-and-valley topography that was produced during a previous 

 and much longer pre-submergence erosional period. On such a shore-line 

 the unconsolidated marine deposits are soon worn away from the headlands 

 during pauses in the emergence, so that unconformable fringing reefs may 

 be formed there. If emergence continue intermittently, the fringing reefs 

 will appear as terraces on the emerged slopes. 



Coasts of this kind appear to be of importance in the coral-reef problem, 

 because they are found to be of frequent occurrence on the deep-water 

 shores of the Australasian region, where unconformable fringing-reef ter- 

 races are reported on many inlands that have embayed shore-lines. It is 

 nevertheless a mistake to conclude, without further question, that all such 

 terracing reefs have been formed during pauses in emergence, although this 

 conclusion has nearly always been adopted by their observers. Such a 

 conclusion tacitly postulates that the previous sulimergence took place at 

 so rapid a rate that no reefs were formed during its progress. Yet it is 

 evidently ecj[ually conceivable that the reefs may have been formed during 

 pauses in a slow submergence and revealed by a rapid emergence (Davis, 

 1916a, p. 499). Discrimination between the two conditions of origin may 

 be made if the structure of the emerged reef is laid bare by erosion, as wall 

 be shown in a later section. In any case, coasts of this kind merit special 

 attention as indicating a pronounced instability. 



Partly Submerged Coasts of Emergence. 



Just as the rule that coasts of emergence are unfavourable to reef-growth 

 is departed from in the case of steep coasts that are partly emerged after a 

 brief submergence, so the rule that coasts of submergence are favourable to 

 reef-growth is departed from in the case of gently sloping coasts that are 

 moderately submerged after a long emergence, during which the adjoining 

 sea-bottom was shoaled by the accumulation of sediments ; for on such 

 coasts the waves will sweep in so much sediment from the shallow bottom 

 that any corals which may for a time attempt to grow on the headlands 

 will soon be smothered and killed. The scarcity of reefs on those islands 

 of the Australasian archipelagoes that have embayed shore-lines fronted by 



