Coral Reefs and Islands. 285 



The current that accompanies the ebb is locally the strongest. 

 Owing to the great width of many barrier-reefs and of the 

 channels and harbours within them, the tide flows in over 

 a wide region. At the turn in the tide the waters escape at 

 first freely over the same wide region ; but, with a tide of 

 but two or three feet, there is but little fall before the reef — 

 which lies at low-tide level and a little above it — retards it by 

 friction ; and thus escape by the open entrances is increased 

 in amount and in rate of flow.- The facts are the same in 

 atolls where the lagoons have entrances*. 



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 j or shear off the projec- 

 ting 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 move- 

 ments 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 platform, has the hardness and solubility in carbonated sea- 

 water of ordinary limestone. 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 becomes 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 50 to 70 yards wide ; 

 at the Port-Jackson Heads, the ordinary tides are six feet high and the 

 platform 50 to 150 yards wide j at the Bay of Islands (in the sheltered 

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

 30 yards wide. 



* The currents of the tropical Pacific Ocean are of very unequal rate in 

 its different parts, and very feeble in the Paumotu Archipelago and the 

 Tahitian and Samoan regions. Capt. Wilkes reports that in the cruise of 

 the Expedition through the Paumotu Archipelago to Tahiti, a distance 

 of a thousand miles, during a month from August 13 to September 13, 

 1839, the drift of the vessels was only 17 miles ; and that during fourteen 

 days in the first half of October, between Tahiti and Upolu of the 

 Samoan group, nearly 1800 miles, the drift was only 43 miles. 



The ' Challenger,' on her route from the Hawaian Islands to Tahiti, found, 

 between the parallel of 10° S. and Tahiti, " the general tendency of the 

 current westerly, but its velocity variable ;" between the parallels of 10° S. 



Phil. Mag. S. 5. Vol. 20. No. 124. Sept. 1885. X 



