REPORT ON THE DEEP-SEA DEPOSITS. 
281 
e. Siliceous Organic Eemains. 
I 
1 , 
I 
I' 
I 
t 
1 
! 
I: 
i' 
'! 
I; 
111 
ii 
i'l 
1' 
ti 
The organisms whose siliceous remains are met with in deep-sea deposits belong to 
three groups ; the Diatomacea and Eadiolaria, both of which have a pelagic habitat, and 
belong to the neritic and oceanic Plankton, and the siliceous Sponges, which live on the 
bottom of the sea, and belong exclusively to the Benthos. Diatoms and Eadiolaria are 
as widely spread throughout the waters of the ocean, and their dead siliceous shells and 
skeletons are as widely distributed over the sea-floor, as the remains of calcareous 
organisms. Siliceous Sponges are also universally distributed on the sea-bed, and their 
skeletons contribute to the materials of marine deposits. The remains of these siliceous 
organisms do not, however, bulk so largely in deep-sea deposits as the calcareous remains, 
still in some regions they are so abundant as to make up a very large part, if not the 
principal part, of a deposit, as, for instance, in the case of Diatom and Eadiolarian Oozes. 
Diatomacea . — These siliceous Algse are met with everywhere in the surface and sub- 
surface waters of the ocean. It is rare, one may say impossible, to drag a very fine tow- 
net through sea- water anywhere without capturing a number of these minute organisms. 
A considerable number of attached forms are carried from land surfaces into the ocean by 
rivers, and in all the shallower depths of the sea such attached forms may be procured, 
but the species that play so large a part in deep-sea deposits are free-swimming and 
pelagic. These pelagic species can generafly be recognised in the tow-net gatherings 
from the sea-surface, if the net used be of very fine texture ; when a coarse net is used 
they can usually be found in the stomachs of the pelagic animals obtained. At times 
they occur near the surface in enormous numbers, in great floating banks many miles in 
extent and several fathoms in depth. When the nets are drawn through these banks 
they are filled with a brown-coloured slimy and felt-like mass, composed principally of 
the frustules of Diatoms. In the tropics the banks are found at the very surface at 
night, and during the day their superior limit may be 10 or 15 fathoms below the sur- 
face. Floating banks of Algae were met with by the ChaEenger in the Southern and 
Antarctic Oceans, in the Sulu Sea, in the Arafura Sea, and off the coast of North 
America, by H.M.S. “Triton” off the Shetland Islands, and in other regions by previous 
and later observers. The dried surface collections made from one of these banks by the 
Challenger in 54° south latitude gave on analysis : ^ — 
Silica soluble in acid, 
Silica insoluble in acid, 
Alumina, 
Organic matter, 
Water, 
1 Made by W. S. Anderson. 
(deep-sea deposits chall. exp. — 1891.) 
I'OO 
76-00 
1-38 
16-75 
4-87 
100-00 
