116 CORAL FORMATIONS. 
in others; and the reefs may be distributed in patches, when without 
such an influence we should expect a general continuity of coral reef 
over the whole reef-grounds. 
The results from marine currents are often increased by waters from 
the island streams; for the coves, where harbours are most likely to 
be found, are also the embouchures of valleys and the streamlets they 
contain. The fresh waters poured in add to the amount of water, and 
increase the rapidity of the out-current. At Apia, Upolu, there is a 
stream thirty yards wide, and many other similar instances might be 
mentioned. ‘These waters from the land bring down also much 
detritus, especially during freshets, and the depositions aid those from 
marine currents in keeping the bottom clear of growing coral. These 
are the principal means by which fresh-water streams contribute 
towards determining the existence of harbours; for little is due to 
their freshening the salt waters of the sea. 
The small influence of the last-mentioned cause—the one most com- 
monly appealed to—will be obvious, when we consider the size of the 
streams of the Pacific islands, and the fact that fresh water is lighter 
than salt, and therefore, instead of sinking, flows on over its surface. 
The deepest rivers 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 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 species of coral 
zoophytes. 
Yet when the rivers are large, like those of continents, the influence 
of the freshening waters is very decided, and prevails often over a 
wide extent of coast. 
Fresh-water streams, acting in all the different modes pointed out, 
are of little importance in harbour-making about the islands of the 
Pacific. The harbours, with scarcely an exception, would have existed 
without them. They tend, however, to keep the bottom more free 
from growing patches of coral, and consequently produce better 
anchorage ground: moreover, within the harbours they usually keep 
channels open sufficiently deep and wide for a boat to reach the shore, 
and sometimes preserve a clean sand-beach throughout. That this is 
their principal effect will appear from a few facts. 
The figure on page 41 has been described as a map of the reef 
