284 Dr. J. D. Dana on the Origin of 



produced by the tides and by rain." Mr. Semper refers to 

 the dead depressed tops of some masses of Porites near tide- 

 level, as the effects of the deposit of sediment over the top of 

 the living coral, and of erosion by the waves and exposure to 

 rains while the sides continued to grow ; and the fact is made 

 an example on a very small scale of atoll-making. Other 

 examples of the action of currents, sediments, boring species, 

 and the solvent action of carbonic acid in the waters are 

 mentioned by Mr. Agassiz, in his excellent account of the 

 " Tortugas and Florida reefs/' 



a. The theory, if satisfactory, accounts not only for the 

 origin of an atoll, but for the origin of atolls of all sizes, 

 shapes, and conditions, and for great numbers of them in 

 archipelagos and chains ; not only for channels through 

 fringing reefs, like those that abrasion in other cases makes, 

 but for all the irregular outlines of barriers, for the great 

 barriers reaching far away from any land, and for the positions 

 and indented coasts of the small included lands. Is it a 

 sufficient explanation of the facts ? 



b. The currents that influence the structure of reefs are : 

 (1) the general movement or drift of the ocean, in some parts 

 varying with seasonal variations in the winds ; (2) the 

 currents connected with wave-action and the inflowing tide 

 over a shelving bottom ; (3) the currents during the ebb, 

 flowing out of channels ; together with (4) counter-currents. 

 Each region must have its special study in order to mark out 

 all the local effects that currents occasion. Such effects are 

 produced whether a secular subsidence is in progress or not, 

 and hence a particular review of the subject in this place is 

 unnecessary. 



The shaping of the outside of the reef and the determination 

 of the width and level surface of the shore platform are due 

 chiefly to the tidal flow and the accompanying action of wind- 

 waves, as explained in § 17 of Part I.* 



* Since the first part of my paper was published I have observed in an 

 article by Mr. A. R. Hunt, in the Scientific Proceedings of the Royal 

 Dublin Society, iv. p. 254, January 1885, the remark, referring to a state- 

 ment of the above fact in my' Manual of Geology/ that the " statement, 

 though strictly in accordance with Mr. Russel's theory, has, so far as I can 

 ascertain, no foundation in fact." The statement, as I have said (and as 

 I illustrate in my ' Geology '), was but the statement of a fact observed by 

 me first in 1839 on the coasts of Australia and New Zealand, without a 

 thought of any theory ; and part of the explanation is overlooked by Mr. 

 Hunt. I observed that the first waters of the incoming tide swelled over 

 the sandstone platform (which was a hundred yards or more wide off the 

 Port-Jackson Heads), and became thus a protector of the sandstone 

 platform from breaker strokes ; and that the lower part of the sandstone 

 bliLff to a height a little above high tide was hollowed out by the strokes 



