231 
1890-91.] Dr Murray and Mr Irvine on Silica in Seas. 
not yield any specimens. Many other examples might be given 
to show that the sponges are clustered in patches over the bed of the 
ocean, with intervening more or less barren spaces. On the whole, 
siliceous sponges are most abundant in moderate depths on the 
Blue Muds along continental shores and in pelagic deposits, and are 
more numerous on Diatom and Badiolarian Oozes than on Globi- 
gerina and Pteropod Oozes. The spicules of sponges frequently show 
signs of undergoing solution, in the widening of the axial canals, 
and the disappearance or thinning of the more delicate processes. 
The spicules contain from 6 to 7 per cent, of water, or in some 
cases as much as 13 per cent.,* associated with organic matter, f and 
belong to the variety of silica known as opal. 
When we turn to the Diatoms and Radiolarians, we find that 
they are universally distributed throughout the surface and sub- 
surface waters, indeed, the tow-net experiments carried out on 
board the “ Challenger ” appear to prove that some species of Radio- 
larians live throughout all the intermediate depths of the ocean. 
Diatoms are abundant in all estuaries and wherever there is a 
low salinity from the admixture of river water, but they are found, 
although more sparingly, in the very saltest waters of the ocean. 
In the waters of the great Southern Ocean and Antarctic regions 
they occur in enormous quantities on the surface, filling the tow-nets 
with a slimy, yellow-brown mass. This slimy mass of siliceous algse 
consisted chiefly of Rhizosolenia, Chcetoceros , and Thalassiothrix , 
and when dried over a stove presented a felted appearance like some 
specimens of asbestos. On analysis this dried mass yielded : — 
Silica soluble in acid, . 
1-00 
Silica insoluble in acid, 
. 76-00 
Alumina, .... 
1-38 
Organic matter, . 
. 16-75 
Water, .... 
4-87 
100-00 
In the Arctic Ocean, and in the seas around the Shetland Islands, 
Diatoms are also at times found in vast floating banks, and they 
* Thoulet, Gomptes Eendus , tome xcviii. p. 1000, 1884. 
t Sollas, Zool. Chall. Exp., part lxiii. pp. 47 et seq. 
