CHAP. \-.] GENERAL CONCLUSIONS. 249 



the mean depth of the Atlantic is a little over 2000 fathoms. 

 An elevated ridge rising to an average height of about 1900 

 fathoms below the surface traverses the basins of the North 

 and South Atlantic in a meridional direction from Cape Fare- 

 well, probably as far south at least as Gough Island, following 

 roughly the outlines of the coasts of the Old and the New 

 Worlds. 



A branch of this elevation strikes off to the south-westward 

 about the parallel of 10° N., and connects it with the coast of 

 South America at Cape Orange ; and another branch crosses the 

 eastern trough, joining the continent of Africa probably about 

 the parallel of 25° S. The Atlantic Ocean is thus divided by 

 the axial ridge and its branches into three basins : an eastern, 

 which extends from the West of Ireland nearly to the Cape of 

 Good Hope, with an average depth along the middle line of 

 2500 fathoms ; a north-western basin, occupying the great east- 

 ern bight of the American continent, with an average depth of 

 3000 fathoms ; and a gulf running up the coast of South Amer- 

 ica as far as Cape Orange, and open to the southward, with a 

 mean depth of 3000 fathoms. 



The Nature of the Bottom. — Except in the neighborhood of 

 coasts, where the deposit at the bottom consists chiefly of the 

 debris washed down by rivers, or produced by the disintegra- 

 tion of the rocks of the coast-line, the bed of the Atlantic, at 

 depths between 400 and 2000 fathoms, is covered with the 

 now well-known calcareous deposit, the globigerina ooze, con- 

 sisting, as has been already described (vol. i., p. 198), to a great 

 extent of the shells, more or less broken and decomposed, of 

 pelagic foraminifera. In the Atlantic the species producing 

 the ooze are chiefly referable to the genera Globigerina, Or- 

 hulina, Pulmnulina, Pullenia, and Sj>h(jeroidina^ the two latter 

 in smaller proportions. 



One very beautiful form occurs at the bottom, sparingly on 

 account of the extreme tenuity of its shell. Ilastigerina Mur- 



II.— 17 



