IS.C. 



IV DEPTHS AND DEPOSITS OF THE OCEAN 135 



with i|- million square miles between 500 and 1000 fathoms. 

 This is due to the large expanses of shallow water in the Arctic 

 regions and Hudson Bay, on the Banks of Newfoundland, off 

 the east coasts of North and South America, between Green- 

 land and the British Isles, around the British Isles, and in 

 the Baltic. 



The most striking feature of the Atlantic Ocean is certainly Mid-Atiamic 

 the low central ridge (dividing the ocean into eastern and '^''^ 

 western deep basins), which was until recently supposed to be 

 continuous from Iceland through both the North and South 

 Atlantic as far as lat. 40° S., but is now known to be discon- 

 tinuous in the neighbourhood of the equator ; on the other hand, 

 it has been extended farther south by the soundings taken on 

 board the "Scotia" in 1904 by Dr. W. S. Bruce, so that the 

 southern limit of the ridge now extends as far south as lat. 

 53" S. At the position of the break in the ridge on the equator 

 the floor of the ocean seems to be more than usually irregular, 

 for depths less than 2000 fathoms alternate with depths exceed- 

 ing 3000 and even 4000 fathoms. On this ridge, with the 

 exception of the Azores group, the only islands are St. Paul's 

 Rocks, Ascension, Tristan da Cunha, and Gough Island. The 

 northern extremity of the ridge between lat. 50° and 60° N. is 

 peculiar because of the number of isolated soundings exceeding 

 2000 fathoms apparently surrounded by shallower water. 



Another point that strikes one in the Atlantic is the gentle Shoie-siopes 

 slope off the American coasts and off the coasts of the British of the Atlantic. 

 Isles, as compared with the slopes off Africa and off Spain and 

 Portugal. This is still more remarkable when compared with 

 the slopes off the Pacific coasts of America. The wide shore 

 platform off the coast of the southern half of South America is 

 especially noteworthy, as well as that off the coasts of the United 

 States and Newfoundland. The shallow area surrounding 

 Rockall Bank also attracts attention. The series of banks made Submarine 

 known as a result of the work of telegraph ships, off the north- A^JlI^nti? ''^^ 

 west coast of Africa to the north of the Canary Islands, is another 

 striking instance of the irregularity of the floor of the Atlantic. 

 In the same neighbourhood the area with depths less than 2000 

 fathoms surrounding Madeira and extending northwards towards 

 the coast of Portugal is remarkable. In the South Atlantic, 

 besides the central ridge, three smaller shallow areas should 

 be noted, two neighbouring ones to the east of the South 

 American coast in lat. 30° S., and the third midway between 

 the ridge and the Cape of Good Hope. 



