20 FOSSIL MEDUSA. 



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 1 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 exjnain 

 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 sal test 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. 3 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. Challemjer; Deep-Sea Deposits, 1891, pp. 28(5-288. 



1 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. 



sphilos. Trans. Royal Soc. Loudon, vol. 176, 1886, pp. 403-448. 



