WYVILLE THOMSON 59 



at depths exceeding 2,000 fathoms was found to be constant 

 at about 36-5° F., while in the N. Pacific the bottom tempera- 

 ture was constant at 35° ; in parts of the S. Atlantic the 

 temperature at the bottom fell to 32-7°, while in the Sulu 

 Sea it is 50*5°, and in the Arafura Sea 38-6°, while it is known 

 that the bottom temperature of the Mediterranean is constant 

 at 55*5°, and that of the Red Sea at 69°, these di£Ferences 

 being due to certain oceanic areas being separated from each 

 other by submarine ridges, which prevent a more general 

 spreading of the cold bottom water from the poles. No 

 bottom temperature was obtained as low as the freezing 

 point of salt water. 



(5) The determination of the exact position of many 

 islands and rocks, the longitude of which had been previously 

 uncertain. 



(6) The charting and surveying of various little-known 

 parts of the world, and their biological investigation. 



(7) The determination of the ocean currents both on the 

 surface and at various depths. 



One of the results of the " Challenger " expedition was 

 undoubtedly an increase in our knowledge of the details of 

 structure and the probable mode of formation of coral reefs 

 and islands. Before the expedition, several geologists and 

 naturalists had published doubts as to the universal applica- 

 bility of the subsidence theory of coral reefs which we owe 

 to Darwin. Semper, for example, showed that in the Pelew 

 Islands up-raised reefs and atolls (which, according to the 

 theory, indicate a sinking area) are found close together. 

 The " Challenger " observations in regard to submarine 

 elevations and the mode of accumulation of deep-sea deposits 

 enabled Mr. Murray (afterwards Sir John) to formulate and 

 publish a new theory as to the origin of atolls, which does not 

 postulate any changes of level, but makes use merely of 

 processes of growth and decay which we know to be at work 

 and constantly acting. The matter is by no means finally 



