184 J. D. Dana — Origin of Coral Beefs and Islands. 



connected with wave-action and the inflowing tide over a 

 shelving bottom ; and (3), the currents during the ebb, flowing 

 out of channels ; together with (1) 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 L* 



The current that accompanies the ebb is locally the strong- 

 est. Owing to the great width of many barrier reefs and of the 

 channels ami harbors within them, the tide flows in over a wide 

 region. At the turn in the tide the waters escape at first freely 



* Since the first part of my paper was published 1 have observed in an article by- 

 Mr. A. E. Hunt, in the Scientific Proceedings of the Royal Dublin Society, iv, 

 254, January, 1885, the remark, referring to a statement of the above fact iu my 

 Manual of Geology, that the " statement though strictly in accordance with Mr. 

 Russell's theory, has so far as I can ascertain, no foundation in fact." The state- 

 ment, 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 Xew 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 bluff to a height a 

 little above high tide, was hollowed out by the strokes of the breakers. A similar 

 erosion near high tide level of the great coral masses standing on the coral-rock 

 platform of atolls I also observed while among the Paumotu Islands. Prof. A. 

 E. Verrill informs me that he has seen examples of the same action on a grand 

 scale about the island of Anticosti in the Gulf of St. Lawrence. The observations 

 do not appear to me to be at variance with the principles laid down in Mr. Hunt's 

 valuable paper ; they require only his recognition of a tidal effect which he does 

 not fully consider, and which British seas cannot illustrate. 



To produce a platform, (1) the rock-material exposed to the flow of the tide and 

 the breakers must be firm enough to resist wear during the early part of the flow, 

 and at the same time soft enough to allow the striking breakers to cut into the 

 base of the bluff, or shear off the projecting ledge ; and (2) the region must not 

 be one of very high tides or stormy seas, for, in such regions of forceful waves 

 and tides, the movements are too often of the destructive kind through the whole 

 continuance of the flow, leaving no chance for the protection a platform needs. 

 Loose sand-deposits are too soft; they are worn off below the sea-level and 

 changed in surface by storms ; but some firmer kinds may make a low-tide flat in 

 a bay where the tides are small. Coral-reef rock, the material of the atoll plat- 

 form, has the hardness and solubility, in carbonated sea-water, of ordinary lime- 

 stone. The rock of the Port Jackson Heads is a friable sandstone. At the Bay 

 of Islands, New Zealand, the platforms occur in an argillaceous rock, which be- 

 comes soft and earthy above by weathering, but is unaltered and firm below 

 because kept wet (loc. cit. p. 442). At the Paumotus the tides are two to three 

 feet high, and the platform usually 100 yards or more wide; at the Phoenix 

 Group the tides are five to six feet high and the platform mostly fifty to seventy 

 yards wide ; at the Port Jackson Heads, the ordinary tides are six feet high and 

 the platform fifty to one hundred and fifty yards wide ; at the Bay of Islands (in 

 the sheltered waters of the bay), the tides are eight feet high and the platform is 

 under thirty yards wide. 



