Roper, on the Diatomacece of the Thames. 79 
obtaining it. Ehrenberg notices an instance where this has 
been done in Jeverland, where a blue sand, abounding in cal- 
careous and siliceous shells, is collected, and greatly increases 
the fertility of the arable soil to which it is applied ; and 
Professor Bailey also states that the mud of Newhaven har- 
bour is used as a fertilizer, and is found to contain 58 '63 
per cent, of silica. 
The distribution of the lower forms of Al^ce, particularly 
the Diatomacece, is probably more extended, both in point of 
time and geographical range, than any other class of or- 
ganized beings. Thus we see associated with gigantic reptiles 
and other extinct forms, several existing species of Diatomacece 
occurring in the chalk formation before the deposition of the 
tertiary strata, proving that the Eocene group is not strictly 
entitled to that designation, but that the dawn of the world in 
which we live extends much further back in the history of our 
planet.* And with respect to their local distribution, Dr. 
Hooker, in alluding to the deposits of the Victoria Bar- 
rier in the Atlantic Ocean, remarks,t "There is probably no 
latitude between Spitzbergen and Victoria Land where some 
of the species of other countries do not exist. Iceland, Britain, 
the Mediterranean Sea, North and South America, all possess 
antarctic Diatomacece. The siliceous coats of species only 
known living in the waters of the South Polar Ocean have 
during past ages contributed to the formation of rocks, and 
thus they outlive several successive generations of organized 
beings. The Phonolite stones of the Rhine, and the Tripoli 
stone, contain species identical with what are now contri- 
buting to form a sedimentary deposit (and perhaps at a future 
period, of rock), extending in one continuous stratum for 400 
miles." 
With the distribution of these forms in our own country 
we are only at present partially acquainted, and the prepara- 
tion, therefore, of carefully-compiled lists of species from 
different localities is still a point to be desired, and might 
probably lead to some interesting generalizations. 
In conclusion, I have only to hope that this slight attempt 
to bring before the Society the results of a careful examination 
of the Thames deposits may induce other and more expe- 
rienced observers to take up the same subject in other locali- 
ties. The facts I have brought forward are sufficient to 
afford, in the words of an excellent observer and late member 
of this Society, " a striking proof of the important part which 
* Humboldt's Cosmos, p. 265. 
t Dr. Hooker, Flora Antarctica, vol. ii., p. 505. 
i Mr. Edwin J. Quekett, in London Physiol. Journ. Feb. 1844, p. 145. 
