232 DALY. 



lagoons. However, certain features of the popular subsidence theory 

 may be reviewed, in order to show more clearly the special advantages 

 of the Glacial-control theory, the only other one involving Recent 

 change of sea-level v/ithin the tropics. Here again discussion will be 

 facilitated by a certain amount of repetition in presenting salient 

 facts. 



Its Alternative Statements. It is important to observe that the 

 Darwin-Dana theory is not the only explanation through subsidence. 

 The vievv' of those famous authors is illustrated in the following passage, 

 taken from the last (1895) edition of Dana's "Manual of Geology,"' 

 page 350. Taking the Pacific area of reefs as a type, he wrote: "If, 

 then, the atolls are registers of subsidence, a vast area has partaken 

 in it — measuring 6,000 miles in length (a fourth of the earth's cir- 

 cumference), and 1,000 to 2,000 in breadth. Just south of the line 

 there are extensive coral reefs; north of it the atolls are large, but they 

 diminish toward the equator, and mostly disappear north of it; and,, 

 as the smaller atolls indicate the greater amount of subsidence, and 

 the absence of islands still more, the line AA [of his map of the ocean] 

 may be regarded as the axial line of this great Pacific subsidence. 

 The amount of this subsidence may be inferred, from the soundings 

 near some of the islands, to be at least 3,000 feet. But as 200 islands 

 have disappeared, and it is probable that some among them were at 

 least as high as the average of existing high islands, the subsidence in 

 some parts cannot be less than 5,000 feet. This sinking probably 

 began in the Tertiary era." 



In postulating a general, prolonged sinking of parts of the sea- 

 bottom, each 10,000,000 to 25,000,000 square km. in area, Darwin and 

 Dana agreed. As to one leading point the principles of their books 

 do not agree. Darwin indicated the possibility of one or more con- 

 siderable pauses in the subsidence; Dana seems not to have considered 

 that suggestion as worthy of emphasis. The necessity of assuming 

 at least one very long pause, if the Darwin-Dana theory is to with- 

 stand even preliminary criticism, will be noted in succeeding pages. 



But Gerland offered a very different version of the subsidence 

 theory. According to him, the coral reefs do not show sinking of wide 

 continuous areas of the ocean floor, but do show the indepenflent 

 sinking of each island mass ("Sockel"). In each case the subsidence 

 is quite local, but has taken place at thousands of different points. ^^ 

 This idea is worthy of attention. Nearly all of the oceanic islands and 



63 G. Gerland, Beitraege zur Geophysik, 2, 56 (1895). 



