76 
UPPER CHALK. 
sent moment, nature, in her secret laboratories, is 
still cariying on a modification of the same opera- 
tion ; of which we have remarkable instances in 
the boiling springs of the Geyser, in Iceland t, and 
of Carlsbad, in Bohemia.! Nor is a high temper- 
ature absolutely essential to the solution of silex 
in water, since this earth occurs in a large propor- 
tion in the mineral waters of our own island* * * § , and 
also enters into the composition of the epidermis of 
various plants of the bamboo tribe, and of the 
English reeds and grasses. The epidermis of the 
Equisetum hyemale, or Dutch rush, consists al- 
most entirely of silex. § 
There is scarcely a single fact in geological 
science, which has given rise to so many unsatis- 
factory conjectures as the formation of the siliceous 
nodules of the chalk. Upon this interesting 
subject, I shall content myself with offering a 
condensed view of the theory proposed by Dr. 
* The deposition of siliceous tufa, or chalcedony, formed by the 
boiling springs of the Geyser in Iceland, are well known : these waters 
contain 31.38 of silex per gallon. Vide Travels in Iceland, by Sir 
George Stewart Mackenzie, Bart. 4to. Edinburgh, p. 389. 
f According to the experiments of Klaproth, the spring at Carlsbad 
contains 25 grains of silex in 1000 cubic inches of water. Mr. De la 
Beche observes, “ We find silex is held in solution by thermal waters, 
which also, as in the case of those of St. Michael’s in the Azores, may 
contain carbonate of lime. No springs, or system of springs, that 
we can imagine, are likely to have produced this great deposit of chalk 
so uniformly over a large surface. But although springs, in our accept- 
ation of the term, could scarcely have caused the effects required, we 
may, perhaps, look to a greater exertion of the agents which now pro- 
duce thermal waters for a possible explanation of the observed pheno- 
mena.” — Geol. Manual, 1st edition. 
J The mineral waters of Bath contain twenty grains of silex in ten 
pints and a half. Nicholson’s Journal, vol. iii. p. 403. 
§ Organic Remains, vol. i. p. 328. 
