230 Proceedings of Royal Society of Edinburgh. [sess. 
cesses at work at the earth’s surface, the carbonic acid replaces the 
silica from its bases, the silica being thus set free to form quartz, or 
a hydrated variety of silica like opal, for example. 
Although there are enormous numbers of organisms in the ocean 
that secrete silica to form their frustules, shells, or skeletons, the 
remains of these siliceous organisms do not play nearly so large a 
part in the formation of the deposits of modern seas as the remains 
of carbonate of lime organisms dealt with in our former paper. 
For our present purpose the siliceous organisms of modern seas 
may be divided into the Sponges, which live on the bottom, or 
belong to the Benthos,* and the Diatoms and Radiolarians, which 
have a pelagic habitat, or belong to the Plankton.* The si- 
liceous sponges belonging to the Tetractinellida, Monaxonida, and 
Hexactinellida, are universally distributed over the floor of the 
ocean, the Hexactinellida being limited to the deep sea, i.e ., to depths 
greater than 100 fathoms. Although universally distributed over 
the ocean’s floor, the spicules of sponges rarely make up over 1 or 2 
per cent, of a deep-sea deposit, except in those limited areas where 
there are extensive patches of these sponges growing on the bottom, 
when the spicules in some samples of a deposit may rise as high as 
20 per cent. At Kerguelen, in 120 fathoms, over one hundred 
specimens of Rossella antarctica were obtained in one haul of the 
trawl ; at Zebu, Philippines, a large number of Euplectella and 
other sponges were obtained in 100 fathoms; off the Ki Islands, 
in 129 fathoms, there were eighteen species of Hexactinellida and 
a large number of individuals; in the Atlantic, near the Cape 
Verdes, there was procured in 1525 fathoms a large specimen of 
Poliopogon amadou (2 x 1 \ feet) attached to the branches of an 
Alcyonarian coral (Pleurocor allium johnsoni) ; off the Kermadecs, 
in 630 fathoms, there was another Poliopogon ( Poliopogon gigas), 
measuring 2 x 3 J feet, and this was but a fragment of what was 
apparently an enormous sponge. In the Faroe Channel large 
numbers of specimens of Pheronema ( Holtenia ) were dredged from a 
depth of 530 fathoms, while trawlings near the same spot did 
* Benthos (fSevdos, bottom of the sea) is a term introduced by Haeckel for 
all those organisms living on or creeping over the bottom of the sea, in contra- 
distinction to Plankton, which, as extended by him, includes all those organ- 
isms swimming about in the sea or carried along in ocean currents ( Plankton - 
Studien , Jena, 1890). 
