90 FOSSIL MEDUS. 
now completely involved and no more distinguishable from one another in the common 
‘“fusionment” than the separate snow crystals of a mass of snow which has been 
frozen by infiltrating water into ice. 
As to the deposition of silica in combination as a silicate, Professor 
Sollas sees no difficulty in the supposition that the dissolved silica derived 
from siliceous organisms should combine with the impurities present in the 
surrounding sediment, and so give rise to glauconitic deposits; thus, with 
such matters as iron oxide, alumina, and potash, the silica is supposed to 
combine, while carbonate of lime merely replaces. 
In discussing the quantity of available silica in the waters of different 
regions, Messrs. Murray and Renard" state that silica was always found 
whenever specially looked for. The analyses, when arranged into a maxi- 
mum set of determinations, show one part of silica in 9,000 to 8,200 parts 
of sea water. When carefully filtered the average proportion from pure 
sea water is one part of silica in 250,000 parts of sea water. This appears 
to be almost constant in purely oceanic waters, coast waters, and in many 
river waters. The amount of soluble silica in sea water is thus so small 
that these authors consider it impossible that this is the exclusive source of 
the silica. They consider the probability of the pelagic organisms which 
secrete silica obtaining it from the hydrated silicate of alumina or clay held 
in suspension as well as the silica held in solution.2 This might explain 
the fact that these organisms abound in brackish waters and waters of low 
salinity and low temperature, where the clay is more abundant than in the 
warmest and saltest waters of the ocean. 
In the case of siliceous sponges, which are rooted for the most part in 
the oozes and clays, Messrs. Murray and Renard think that the silica of their 
skeletons may be derived from the silica in solution in sea water, or from 
the colloid silica set free during the decomposition of the feldspathic rock 
fragments and minerals in the deposits. 
In an article on beds of sponge remains in the Lower and Upper 
Greensand of the south of England, Dr. Hinde gives an interesting account 
of the mode of occurrence of the cherts in England, Germany, France, 
and Belgium.’ In the discussion of the mineral conditions of the sponge 
remains and the beds derived from them, Dr. Hinde states that no fossil 
1 Rept. Voyage H. M.S. Challenger; Deep-Sea Deposits, 1891, pp. 286-288. 
2 See also Murray and Irvine, On silica and the siliceous remains of organisms in modern seas: 
Proc. Royal Soc. Edinburgh, Vol. XVIII, 1891, pp. 246-250. 
’ Philos. Trans, Royal Soc. London, vol. 176, 1886, pp. 403-448. 
