THEORIES OF ORIGIN OF REEFS 409 



some evidence of elevation. The subsidence theory accounts for 

 this distribution by assuming that the central area has gone down 

 so fast that the corals could not gain a foothold or were drowned 

 out completely. The small atolls show a rapid and great sub- 

 mergence, but not sufificient to prevent coral growth entirely, the 

 upward growth barely keeping pace with the subsidence, and with 

 no opportunity for spreading. The successive belts show the de- 

 crease in the amount of subsidence, so that we pass progressively 

 from large atolls to barrier reefs and finally to fringing reefs, where 

 the zone of no subsidence is reached, beyond which the movement 

 seems to have been in the opposite direction and dead, fringing 

 reefs cling to the sides of high islands often at a considerable ele- 

 vation above the present sea-level. 



Evidence of subsidence. The most convincing proof of the cor- 

 rectness of this theory for the explanation of at least some of the 

 coral reefs of the Pacific is furnished by the deep boring on the 

 atoll of Funafuti, which showed organic limestone consisting of 

 corals, Foraminifera, calcareous alg?e and other organisms to a 

 depth of 1,114 feet, the extent of the boring. The boring made in 

 the lagoon showed the existence of solid masses of shallow-water 

 corals (Porites, Madrepora, Heliopora) at a depth of 245 feet un- 

 der a layer of calcareous algae {Halimcda opuntia) and Forami- 

 nifera, more than a hundred feet thick. Other evidence of sub- 

 sidence is furnished by sacred structures erected by the natives on 

 certain coral islands. These now stand in the water, and the paths 

 worn by the feet of the devotees are at present passages for ca- 

 noes. (Dana-20.) 



Davis has recently shown (27,) that Dana furnished con- 

 vincing proof of the theory of subsidence in the region of some 

 coral atolls. This lies in the fjord-like bays which indent much 

 eroded mountain islands, around which coral reefs grow. If no 

 subsidence has taken place, these valleys would be more or less 

 filled by alluvial deposits, since clastic material is constantly brought 

 into the sea by the wash from the mountain. Where, however, the 

 valleys are unobstructed embayments, and continue as such under the 

 water, the evidence of progressive subsidence with corresponding en- 

 croachment of the sea up the valleys seems clear. As Dana says : 

 "The very features of the land, the deep indentations, are sufficient 

 evidence of subsidence to one who has studied the character of the. 

 Pacific islands." And, again : "When . . . we find the several val- 

 leys continued on beneath the sea, and their enclosing ridges stand- 

 ing out in long, narrow points, there is reason to expect that the land 

 has subsided after the formation of the valleys. For Such an island 



