57 
perhaps, be reniinded that the cuticle of the Dutch rush and the 
stellate hairs of Deutzia, both of which are silicious, do polarize, and 
exhibit brilliant colour when examined by light in that condition, 
but this is not in consecpience of their silicious nature, but is due 
to the presence of a membraneous film investing the cuticle or 
hair. If a piece of Ecjuisetum or Deutzia is boiled in sulphuric 
acid, and then decarbonized with chlorate of potash, a display of 
colour will no longer be visible. The shells of the polycystina, 
spo]ige.s, spicules, and the Diatornaceie are all singly refractive. 
The base of silica is Silicon. Silica under certain conditions is 
soluble in water to a considerable extent. "Waters holding silica 
in solution, that is to say, in any large (luantity of it, are now 
extremely rare; the Geyser and llykniii in Iceland, and the 
Pennakoon and Loongootha in India are the best known. An 
analysis of a gallon of the Geyser water showed 31-50 of silica. 
It is highly probable that silica was present in larger <juantitics in 
the earlier epochs of tlio world. This, however, is not a (piestion 
of much importance, as we know that it exi.sts in a soluble form, 
and is eliminated, often in great abundance, by various organisms. 
I need only refer you to the Alcyoncellum, Ilyaloneina, Idiaro- 
nema, and other Silicious Sponges as evidences of that fact. 
The ])resence of silica in a state of solution being an a.scertained 
fact, there is nothing improbable in the hypothesi.s, that sponges 
should have formed the nuclei of these flinty concretioms, the 
silicious spicules would possibly exert an attractive influence upon 
the atoms of silica in solution, in a similar way that a crystal of 
saltpetre Avould be the starting point for further crystallization in a 
solution of that .salt. 
Another and still more etfectual cause of the elimination of silica 
would be tlie decomposition of the sarcode and keratode material, 
as this goes on certain gases are produced, and the silex precipitated 
from a solution. 
The discoveries of Wallich, Carpenter, and others, of protoplasm 
or sarcode existing at the bottom of the ocean, and to which I 
have previously alluded, appear to afibrd a probable explanation of 
the cause of solution to the flinty layers found in some of the 
chalk strata. The opponeijts to the spongeous origin of flints 
brought forward the the existence of these layers as a proof that 
sponges were not tlie nuclei of Flints, and until the existence of 
