Basin of the Indian Ocean. 19 



and Trinidad. They are divided from each other in lat. io° 

 N. by a submarine elevation, which apparently connects the 

 central plateau with the South American continent. The 

 supposed existence of this dividing ridge is founded, not so 

 much upon soundings, which are very few in this part of the 

 North Atlantic, as upon the difference of bottom-temperature 

 observed in the two depressions. The higher temperatures 

 ascertained by the " Challenger " at the bottom of the basin 

 north of the Equator seem to indicate that the inflow of cold 

 water from the southern basin is arrested. The distance 

 between the observing-stations, amounting to about 20° of lat. 

 however, is sufficiently great to justify the conclusion that the 

 difference of the bottom-temperature in the two areas of 

 depression, which does not exceed i° C. may be due to a 

 difference of latitude, since the gradual increase from South to 

 North of the bottom- temperature is one of the characteristic 

 features of the Atlantic. 



The Basin of the Indian Ocean. — The soundings taken 

 in this ocean prove the existence of a submerged plateau on the 

 limit between the Indian Ocean and the Southern Ocean. It 

 rises in many parts to within 1500 fathoms of the sea-surface, 

 and forms the common foundation of all the islands situated 

 in this part of the world — viz., Prince Edward Islands, the 

 Crozet Islands, the Kerguelen group, the Heard Islands, and 

 the islands of St. Paul and Amsterdam. The origin of all these 

 islands is probably volcanic. The plateau occupies the space 

 comprised between the meridians of 35° and 8o° long. E., and 

 the parallels of lat. 35 and 55 S., and the soundings 

 obtained by H.M.S. "Challenger" between the Heard Islands 

 and the Antarctic Circle establish a connection with the 

 Antarctic plateau, of which the above plateau seems to be an 

 extension towards lower latitudes. 



The main basin of the Indian Ocean, with an average depth 



