34 On Coral Reefs and Islands. 
Art. VI.—On Coral Reefs and Islands; by James D. Dana 
Part Fifth. 
From the Report on Geology of the Exploring Expedition under Capt. Wilkes, U.S.N. 
2. Causes modifying the Forms and Growth of Reefs. 
Corat reefs although (1) dependent on the configuration of the 
submarine lands for many of their features, undergo. various 
modifications of form or condition through the influence of ex- 
traneous causes, such as (2) unequal exposure to the waves; (3) 
oceanic or local currents ; oe — y. Sresh or impure waters. 
In_ briefly treating of these ics, we may consider first, reefs 
around high islands, and whens atoll reefs. The effect ‘of the 
waves on ‘different sides of reefs has already been considered, and 
“we pass on, therefore, at once to the influence of oceanic or local 
currents, and fresh or eur waters. 
and Fringing Reefs—The evistence of harbors 
about weal Bound lands, and of entrances through reefs, is largely 
attributable to the action of tidal or local marine currents. 'T 
presence of freshwater streams has some weet towards the same 
end, but much less than has been su 
There are usually strong tidal eeheeeiie ee the reef chan- 
nels and openings. ‘These currents are modified in character by 
coves or bays to receive the advancing tides. The harbor of 
Apia, on the north side of Upolu, affords a striking illustration of 
this general principle. The coast at this place has an indenta- 
tion 2000 yards wide and nearly 1000 deep, as in the accompany- 
ing sketch, reduced from the chart b : 
Expedition. The reef extends from either 
side or cape a mile out to sea, leaving 
between, an entrance for ships. The har- 
bor averages ten feet in depth, and at the | 
entrance is iffecn feet. In this harbor oe 
more modaanie weather the | eds may tail out pe the wind. 
Thus when no current but one inward is perceived at the surface; 
there is an under current acting against the keel and bottom of the 
p— vessel, which is of sufficient strength to counteract the influence — 
of the winds on the rigging and hull. The cause of such acu — 
io cna ae eae z 
* The view here supported, is nearly identical with that present Mr. Dar 
win, (op. cit., page 68.) The argume rues given were, however, por a in 1840, 
before his eg ee of coral islands were known to me. This fact may give be" 3 
ditional weight to the opinions, reach they are there conelusions of 
and are substantiated by a distinct set of observations. ; 
