288 THREE CRUISES OF THE " BLAKE." 



in addition, sweeps the floor of the plateau clear of all fine sedi- 

 mentary accumulations.^ 



" A similar sea-bottom, but with a very steep slope, is formed 

 on the north side of Cuba, to a depth of three or four hundred 

 fathoms, also inhabited by a rich fauna, which presents, how- 

 ever, considerable differences from the one just mentioned, not- 

 withstanding the short distance between the two coasts." (Pour- 

 tales.) 



Near the Bahama Banks, Pourtales found the steep slope cov- 

 ered with calcareous sand. In his description of the fauna of 

 these plateaux he called attention to the resemblance of many of 

 the types described by him to tertiary and cretaceous forms, and 

 to their extended geographical distribution. 



Mr. Murray thus describes the typical bottom deposits of the 

 Caribbean district : — 



" The specimens procured around the shores of the Greater and Lesser 

 Antilles are chiefly fi"om depths between a hundred and one thousand 

 fathoms, although a few are in depths less than a hundred fathoms 

 and a few are over two thousand fathoms. They are all in more 

 or less close proximity to the coasts. The mineral particles are chiefly 



^ The hard parts of the corals, mol- these regions, are frequently coated with 



lusks, sponges, and other invertebrates, a glaze of manganese, 

 which build up the limestone of the Pour- Sharpies found traces of oxide of iron 



tales Plateau, are often riddled with ca- in the ooze he analyzed from the Straits 



nals made by vegetable parasites, or by of Florida, and a considerable quantity 



boriug sponges. The former often form in the concretions and specimens of rock 



beautiful networks. Their manner of of the Pourtales Plateau. The presence 



boring is not well understood. Wedl and of iron in a closed sea, like the Gulf of 



KoUiker suggest that the vegetable para- Mexico, is readily accounted for by the 



sites may dissolve the carbonate of lime presence of large rivers carrying down 



by emitting an acid as they advance, at into it large quantities of iron. The 



their terminal extremity. In the case of waters from the open ocean are nearly 



boring sponges, the penetration is proba- free from it. The following are Sharples's 



bly mechanical. The canals, like the sur- analyses : — 

 face of many of the concretions from 



Rock Concretion. Rock and Corals. Limestone. Ooze. 



Carbonate of lime . . . .36.50 47.11 96.96 85.62 



Tricalcic phosphate . . . .35.54 13.15 1.20 .18 



Silica 49 1.9'2 2.12 1.52 



Ferric oxide 14.77 20.23 — .31 



Carbonate of magnesia . 10.56 12.39 — 4.26 



Organic matter and water . 1.46 5.89 — 8.15 



99.32 100.69 100.28 100.04 



