FORMA TIOiV OF CORAL REEFS AND ISLANDS. 207 



by the detritus which they deposit, 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 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 map of the reef of North Tahiti, between Papieti on the 

 left, and the west cape of Matavai harbour, on the right, 

 here reproduced, affords illustrations of this subject. 



a. The harbour of Papieti is inclosed 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 harbour receives an unimportant streamlet, while a 

 much larger stream empties itself just to the east of the east 

 cape, opposite lu/iick the reef is close at hand and imbroken. 



b. Toanoa is the harbour 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, harbour-like in 

 character, but is without any entrance ; the reef is unbroken. 

 Yet there are two streams emptying into it, one of which is of 

 considerable size. 



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

 for about two miles. The harbour 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 

 toward Point Venus, and at the present time the waters find 

 theii 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 about the Pacific Islands by fresh-water 

 streams. We cannot be surprised at the little influence they 

 appear to have exerted about Tahiti, when knowing that none 

 of these so-called rivers are over three feet in depth ; and the 



