1888.] on Structure, Origin^ and Distribution of Coral Beefs, dc. 253 



CTiallenger sounded along the west coast of Africa, there was no 

 suspicion that between her stations she was sailing over submerged 

 cones. Since then, however, the soundings of telegraph ships have 

 correctly mapped out no less than seven of these peaks between the 

 latitude of Lisbon and the island of Teneriffe. The depths on the 

 summits of these vary from 12 to 500 fathoms. On one of them, at 

 400 fathoms, two species of coral ( Lo_phoheIia jjrolifera and AmjpJiiheUa 

 oculafa) were growing luxuriantly. Throughout the ocean basins 

 about 300 such submarine cones, rising from great depths up to 

 withiQ depths of from 500 to 10 fathoms from the surface, are 

 already known, or indicated by soundings. 



All the agencies at work above the lower limit of wave action 

 tend to wear away and level down these cones, and thus to form 

 banks. Graham's Island, thrown up in the Mediterranean in 1831, 

 was 200 feet in height and three miles in circumference, and was 

 washed away in a year or two. The bank left on the spot, at first 

 very shallow, has now 21 feet of water over it. Instances similar to 

 this historical example must often have happened in the great ocean 

 basLQS. Again, the same agencies produce wide banks around vol- 

 canic islands by washing away and spreading out the materials of 

 the softer rocks. Such banks, with depths of less than 60 fathoms, 

 are found extending many miles seawards around some volcanic 

 islands. 



On the other hand, all the deeply submerged summits are con- 

 tinually being built up to the lower limit of wave action by the accu- 

 mulation of the remains of animals which live on them and by the 

 fall of shells upon them from the sui'face waters. In the Solomon 

 Islands, Dr. Brougham Guppy has shown that there are upraised 

 coral islands with central volcanic cones covered with thick layers of 

 marine deposits ; Christmas Island in the Indian Ocean is another 

 instance, and similar deposits must now be forming over hundreds of 

 submerged mountains. In this way are foundations prepared for the 

 true reef-building species, which only flourish in the shallower 

 depths. 



The bulk of the water of the ocean has a very low temperature : 

 it is ice-cold at the bottom, even under the equator, but on the surface 

 withia the tropics, there is a relatively thin film of warm water, with 

 a temperature of from 70" to 84^ Fahr. This film of warm water 

 is much deeper towards the western parts of the Atlantic and Pacific 

 than it is in the eastern, the reason for this being that the trade 

 winds, which blow contiaually from the east, carry all the warm 

 surface water to the westward, and draw up cold water from beneath 

 along the western shores of Africa and America to supply the place 

 of that driven westward at the surface. Consequently, there is, at 

 times, a very low temperature, and a great annual range of tempe- 

 rature, along these western shores. This is more clearly shown by 

 the temperatures at 50 and 100 fathoms than by those at the sur- 

 face. There are no coral reefe along the western shores of Africa and 



