98 ON CORAL REEFS AND ISLANDS. 



down upon the chart of the world accompanying my Report. 

 In the Pacific it is observed to exclude the Galapagos,* and reach 

 the South American coast north of the equator, iustead of at the 

 parallel of 28° south, the position in mid-ocean. On the coast 

 of Asia it curves from the equator beyond latitude 30°. In the 

 Atlantic it forms an abrupt bend far to the north, in the line of 

 the Gulf Stream, and includes the Bermudas in latitude 32° N. ; 

 while on the African coast the northern line curves downward to 

 the latitude of the (."ape V r erds, and the southern upward nearly 

 to the equator. The following table will give more definitely 

 the position of the coral boundary line where it meets the coasts 

 of the continents. 



Pacific Ocean. Atlantic Ocean. 



East side of ocean— Northern, Latitude 21° N". Latitude 10° N. 



Southern. 4°N. 5° S. 



West side of ocean— Northern. 34° N. 34° N. 



Southern. \%%- New Holland. ) , 2 c s 



I 29° S., Atnca. ) 



It follows from the above, that while the coral-reef seas are 

 about fifty-six degrees wide in mid-ocean, they are in the Pacific 

 seventeen degrees wide on the west coast of America, and sixty- 

 four degrees on the Asiatic side. In the Atlantic, they are about 

 fifteen degrees wide on the African coast, and fifty-six degrees 

 on the coast of America. If we reckon to the extremity of the 

 bend in the Gulf Stream, the whole width off the east coast of 

 America, north of the equator, will be over forty degrees. It is 

 obvious that these facts enable us to explain many seeming 

 anomalies in the distribution of coral reefs. 



Within the limits included by the coral-reef boundary line, 

 those other causes operate which influence the distribution of 

 reefs. The effect of a deep abrupt coast has been pointed out. 

 The unfavorable character of sandy or muddy shores, and the 

 action of detritus, marine currents, and fresh waters have also 

 been stated. 



No less striking are the effects of volcanic, action in preventing 

 the formation of reefs; and instances of this influence are nu- 

 merous throughout the Pacific. The existence of narrow reefs, 

 or their entire absence, may often be thus accounted for. For 

 example, in the Sandwich Group, the island Hawaii, still active 

 with volcanic fires, has but (e.w traces of corals about it, while 

 the westernmost islands, which have been longest free from such 

 action, have reefs of considerable extent. The island of Maui 



* Captain Fitzroy, R.N., found the surface temperature of the sea at the Galapa- 

 gos, from Sept. 16 to Oct. 18, 1835, 62° to 70° F. Oct. 23, in lat. 0° 30' S., and 

 long. 99° 4' W., the temperature of the sea was 66° F. Oct. 24, lat. 0° 23' N., 

 long. 96° 53' W., temp. 10%°, 7H° F. While under the equator, about the middle 

 of the Pacific, the range of surface temperature of the sea through the year is 81° 

 to 88° F. 



