origin, organic contents, and other characters which belong to them as aqueous
formations, independently of their age, and we may afterwards consider them
chronologically or with reference to the successive geological periods when they
originated.

I have already given an outline of the data which led to the belief that the
stratified and fossiliferous rocks were originally deposited under water; but,
before entering into a more detailed investigation, it will be desirable to say
something of the ordinary materials of which such strata are composed. These may
be said to belong principally to three divisions, the siliceous, the
argillaceous, and the calcareous, which are formed respectively of flint, clay,
and carbonate of lime. Of these, the siliceous are chiefly made up of sand or
flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter
with a certain proportion, about a fourth in weight, of aluminous earth; and,
lastly, the calcareous rocks, or limestones, of carbonic acid and lime.

SILICEOUS AND ARENACEOUS ROCKS.

To speak first of the sandy division: beds of loose sand are frequently met
with, of which the grains consist entirely of silex, which term comprehends all
purely siliceous minerals, as quartz and common flint. Quartz is silex in its
purest form. Flint usually contains some admixture of alumina and oxide of iron.
The siliceous grains in sand are usually rounded, as if by the action of running
water. Sandstone is an aggregate of such grains, which often cohere together
without any visible cement, but more commonly are bound together by a slight
quantity of siliceous or calcareous matter, or by oxide of iron or clay.

Pure siliceous rocks may be known by not effervescing when a drop of nitric,
sulphuric or other acid is applied to them, or by the grains not being readily
scratched or broken by ordinary pressure. In nature there is every intermediate