18 Depth of the Ocean. 



Island, where, resuming its southward course, it widens out 

 until in lat 30° S. it occupies nearly half the space between 

 South America and Africa, uniting the island of Ascension 

 with St. Helena in the east, Trinidad in the west, and the 

 group of Tristan d'Acunha and Gough Island at its southern 

 end. In the absence of soundings to the southward of Gough 

 Island, it is difficult to form an opinion as to whether this 

 plateau connects itself by its southern end with the Antarctic 

 plateau, or whether it is separated from the latter by an area of 

 depression extending along the parallel of lat. 50° S. from the 

 Falkland Islands to the meridian of the Cape of Good Hope. 

 Certain indications derived from the nature of the currents, and 

 from the deep-sea temperatures observed in that region of the 

 South Atlantic, are in favour of the latter hypothesis. 



Considerable portions of this plateau are within 1500 

 fathoms, or a mile and a-half, of the surface of the sea, and 

 most of the islands are of volcanic origin. An extinct volcano, 

 8300 feet in height, forms the island of Tristan d'Acunha; 

 Ascension Island rises to 2800 feet, and the summit of Pico 

 in the Azores to 7600 feet above the level of the sea. The 

 northern end of the plateau joins, as already stated, the plateau 

 of Iceland with its still active focus of eruption. 



By this central plateau, the Atlantic Ocean is divided 

 into two longitudinal areas of depression or channels, one 

 following the shores of North and South America, the other 

 the shores of Europe and Africa. The depths vary from 

 2000 fathoms to nearly 4000 fathoms, the average depth 

 being about three miles. The deepest portion of the eastern 

 channel is situated to the westward of the Cape de Verde 

 Islands, and forms an area of depression of over 3000 

 fathoms (Plate 3). In the western channel there are two 

 such depressions, one placed between the Antilles, Bermudas, 

 and the Azores, the other between Cape St. Roque, Ascension, 



