CAUSES MODIFYING THE FORMS AND GROWTH OF REEFS. 81 



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, and prevails 

 often over a wide extent of 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 scarcely 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 14, has been described as a map of the 

 reef of North Tahiti, between Papieti on the left, and Venus 

 Point on the right. 



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 center, 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 entrance 

 is thirty-five fathoms deep at middle, and three and a half to five 

 fathoms near the points of reef. There is no freshwater stream, 

 excepting a trifling rivulet. 



c. Papaoa 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 emptying into it, one of which is of con- 

 siderable size. 



d. Off Matavai, the place next east, the reef is interrupted for 

 about two miles. The harbor is formed by an extension of the 

 reef off Point Venus, the east cape. There is no stream on the 

 coast, opposite this interruption 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. 



From such facts, it is evident that the growth of coral reefs is 

 not much retarded by freshwater streams. We cannot be sur- 

 prised at the little influence they appear to have exerted when 



11 



