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- 
talés.) 
Near the Bahama Banks, Pourtalés found the steep slope cov- 
ered with calcareous sand. In his description of the fauna of 
these plateaux be 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 from 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 
1 The hard parts of the corals, mol- these regions, are frequently coated with 
lusks, sponges, and other invertebrates, 
which build up the limestone of the Pour- 
tales Plateau, are often riddled with ca- 
nals made by vegetable parasites, or by 
boring sponges. The former often form 
beautiful networks. Their 
boring is not well understood. Wedl and 
Kölliker suggest that the vegetable para- 
sites may dissolve the carbonate of lime 
by emitting an acid as they advance, at 
their terminal extremity. 
boring sponges, the penetration is proba- 
bly mechanical. The canals, like the sur- 
face of many of the coneretions from 
manner of 
In the case of 
Rock Coneretion, 
Carbonate of lime 36.50 
Tricalcie phosphate ‚ 95.54 
Silica i 490 
Ferric oxide . j e 1474 
Carbonate of magnesia 10.56 
Organic matter and water . 1.46 
99,32 
a glaze of manganese. 
Sharples found traces of oxide of iron 
in the ooze he analyzed from the Straits 
of Florida, and a considerable quantity 
in the concretions and specimens of rock 
of the Pourtalès Plateau. 
of iron in a closed sea, like the Gulf of 
Mexico, is readily accounted for by the 
presence of large rivers carrying down 
'The 
waters from the open ocean are nearly 
free from it. The following are Sharples's 
The presence 
into it large quantities of iron. 
analyses : — 
ock and Corals, Limestone. Ooze. 
47.11 96.96 85.62 
13.15 1.20 18 
1.92 2.12 1.52 
20.23 — д1 
12.89 -— 4.26 
5.89 — 8.15 
100.69 100,28 100,04 
