DAG CORALS AND CORAL ISLANDS. 
are seldom over six feet, even at their mouths; and three or 
four feet is a more usual depth. They will have little effect, 
therefore, on the sea water beneath this depth, for they can- 
not sink below it; and corals may consequently grow even 
in front of a river’s mouth. 
Fresh-water 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 scarcely an ex- 
ception, would have existed without them. They tend, how- 
ever, by the detritus which they deposit, to keep the bottom 
more free from growing patches of coral, and keep channels 
over the shore reef sutticiently deep and wide for a boat to 
reach the land. 
The map of the reef of North Tahiti, on page 149, and 
the following map of Matavai Bay on a larger scale, afford 
illustrations of this subject. 
a. The harbor of Papieti is enclosed by a reef about three 
fourths of a mile from the shore. The entrance through the 
reef is narrow, with a depth of eleven fathoms at centre, six 
to 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 the east cape, 
opposite which the reef is close at hand and unbroken. 
b. Toanoa is the harbor next east of Papieti. The en- 
trance is thirty-five fathoms deep at middle. and three and 
a half to five fathoms near the points of the reef. There is 
no fresh-water stream, except a trifling rivulet. 
c. Papaoa is an open expanse of water, harbor-lke in 
character, but without any entrance; the reef is unbroken. 
Yet two streams empty into it. 
d. Off Matavai, the place next east, the reef is inter- 
rupted for about two miles. The harbor is formed by an 
