246 PHYSICAL GEOLOGY 



not end in sea cliffs, as would be the case if a volcanic island 

 had been cut back by the waves to form a marine platform upon 

 which the corals grew. (4) It has also been found that bor- 

 ings of 1000 or more feet penetrate materials like those of the 

 superficial layers of the reef. Also, according to this theory, the 

 barrier reef gradually contracts as subsidence continues, resulting, 

 if the sinking has been long continued, in the complete drowning 

 of the island and the formation of an atoll and finally of a lagoonless 

 island. 1 



Submarine Bank Theory of Murray and Others. — This theory 

 holds that barrier reefs and atolls may be explained without postu- 

 lating subsidence of the sea floor. The supporters of this theory be- 

 lieve that banks may be built up by the accumulation of the remains 

 of marine animals until a depth of water suitable for coral growth is 

 attained, or a platform for the corals may be formed by volcanic 

 cinder cones (such as that of Graham's Island). 



Since coral growth is most rapid on the outer margin of such a bank 

 or cone, a ring arises with a lagoon within. Waves break through the 

 ring, separating it into a series of islets, and the solvent action of sea 

 water together with the erosion of currents deepens and widens the 

 lagoon. According to this theory the coral ring grows larger; ac- 

 cording to the subsidence theory it becomes continually smaller. In 

 support of this theory it is pointed out that elevated atolls are some- 

 times mere skins on older volcanic rocks and are not of great thick- 

 ness. In many cases, moreover, coral atolls rest upon limestone and 

 volcanic rock which have been cut down by erosion and have not 

 sunk. The most serious objection to this theory is that sediments 

 carried into the lagoons by streams from the islands have not built 

 delta plains, as would be the case had the region suffered no subsid- 

 ences. Some atolls have probably been formed in this way, but the 

 general application of the theory is not justified. 



Change in Sea Level Due to Glaciation, or the Glacial-control 

 Theory. — An ingenious theory elaborated by Daly is based upon 

 the lowering of the level of the sea in the tropics, due to the with- 

 drawal of the water in those regions by evaporation, and its later 

 precipitation in the north as snow during the formation of the great 

 'Pleistocene) ice sheets; this withdrawal being increased by the 

 attraction of the water by the great mass of ice in the Arctic 

 regions. '* The ice sheets (Pleistocene) which have since melted 



1 Davis, W. M., — Nature, Vol. 90, 1913, pp. 632-634. 



