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



coasts of more ancient emergence are also generally ill adapted to reef 

 establishment, because as long as they stand still the frontal slopes of the 

 deltas that are built forward and the broad surface of the platforms that 

 are cut backward are overspread with shifting detritus, on which corals 

 can not attach themselves. Moreover, in virtue of coastal emergence, the 

 rivers of the region will, as a rule, be revived to more active erosion than 

 before; the detritus that they discharge, added to that spread from the 

 cliffs by the waves, will cloak the platform with loose seaiments and pre- 

 vent coral growth. The following examples may be cited: 



Part of the southwest coast of Sumatra is, according to Erb (1905), a 

 coastal plain of Pliocene strata, bordered by a beach of gravel beaten by 

 heavy surf and drifted northwestward by the long-shore current; dis- 

 continuous fringing reefs are found near the southeastern end of the 

 coast, where the gravel beach is less developed, and some of them are at- 

 tached to the eroded margin of uplifted reefs which stand 20 or 30 me- 

 ters above sealevel and seem to offer a firm support for the new growths. 

 The southwest coast of Java as described by G-uppy consists of a coastal 

 plain of gently inclined foraminiferous tuffs, and seems to offer an ex- 

 ample of a simple shoreline of emergence in a later stage of development 

 than that of Java, for it has been retrograded so that the emerged strata 

 are cut off in a bluff, 40 or 50 feet in height (1889, a). Beneath the bluff 

 lies a beach of dark volcanic sand, on which the heavy surf breaks unre- 

 strained by coral reefs. But fringing reefs do occur on short stretches of 

 the shore, and "there was a time, in fact, when a large portion of this 

 coast was fronted by shore reefs, which have since been killed and over- 

 whelmed by the great quantities of sand and mud brought down by the 

 rivers (1889, h, 630). The last statement seems to justify one of the 

 principles adopted above in the section on the development of clift vol- 

 canic islands in the coral seas. 



Borneo appears to be, according to Molengraaff, reef -free around most 

 of its shoreline, because of the abundant outwash of detritus since its late 

 Tertiary uplift, whereby extensive alluvial deltas have been built forward 

 from the margin of its emerged coastal plain; it is "a worn, much de- 

 nuded mountain land, surrounded for the greater part by broad tracts of 

 low land, covered with fluviatile deposits . . . the result of prolonged 

 and intense erosion of a mountainous island surrounded by a shallow sea" 

 (1902, 453). Coral reefs seem to have been absent previous to the late 

 Tertiary uplift as well as now, for the mass that then emerged is described 

 as an older nucleus unconformably surrounded by nearly horizontal coal- 

 bearing strata, which were presumably formed under conditions very 

 similar to those obtaining on the aggraded fluviatile lowland of today. 



