36 On Coral Reefs and Islands. 
more usual depth. They will have little effect, therefore, on the 
sea water beneath this depth, for they cannot sink below it; and 
corals may consequently grow even in front of a river’s mouth. 
Moreover, the river water becomes mingled with the salt, and, in 
most cases, a short distance out, would not be unfit for some spe- 
cies of coral zoophytes 
Yet when the rivers are large, like those of continents, the in- 
fluence of the freshening waters is very decided, aud prevails — 
often over a wide extent al coast. : 
Freshwater streams, acting in all the different modes pointed 
out, are of little importance in harbor-making about the islands — 
of the Pacific. The harbors, with scareely an exception, would — 
have existed without them. They tend, however, by the detritus — 
they deposit, to keep the bottom more free from growing patches 
of coral, and consequently produce better anchorage ground: more- 
over, within the harbors they usually keep channels open through — 
or over the shore reef sufficiently deep and wide for a boat to reach 
the land, and sometimes preserve a clean sand-beach throughout. — 
That this is their principal effect will appear from a few facts. 
The figure on page 366, vol. xi, of this Journal, has been de- 
scribed as a map of the reef of North EN between Papieti 
on Lag left, and Venus Point on the right. 
he harbor of Papieti is enclosed by a reef about three 
iueasial of a mile from the shore. The entrance through the 
reef is narrow, with a depth of eleven fathoms at center, six t0 — 
seven fathoms either side, and three to five close to the reef. 
This fine harbor receives an unimportant streamlet, while a much 
larger stream empties just to the east of - east CAPS; opposite — 
pan the reef is close at hand and unbro 
Toanoa is the harbor next east of Pap ieti. The entrance 
is tines fathoms deep at middle, and three and a half to five 
fathoms near the points of reef.. There is no freshwater stream, 
omer ee a trifling rivulet. 
apaoa is an open expanse of water, harbor-like in charac- 
ter, "but is without any entrance; the reef is unbroken. Yet 
there are two streams ein into it, one of which is of con- 
siderable siz 
d. Of etasel the place next east, the reef is interrupted for 
about two miles. The polos is fonmesd by an extension 0 of the 
reef off Point Venus, the cape. There is no stream on the 
coast, opposite this intberuptiog in the reef, except towards Point 
Venus, and at the present time the waters find their principal 
exit, east of the Point, behind a large coral reef, but a quarter of 
a mile distant i 
From such forts, it would almost seem is it coral reefs gre” : 
best near freshwater streams. We be surprised at the 
little influence they appear to hase exerted when — that 6 
