36 
ELEMENTS OF GEOLOGY. 
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 bro¬ 
ken by ordinary pressure. In nature there is every inter¬ 
mediate gradation, from perfectly loose sand to the hardest 
sandstone. In micaceous sandstones mica is very abundant; 
and the thin silvery plates into which that mineral divides 
are often arranged in layers parallel to the planes of stratifi¬ 
cation, giving a slaty or laminated texture to the rock. 
When sandstone is coarse-grained, it is usually called grit. 
If the grains are rounded, and large enough to be called peb¬ 
bles, it becomes a conglomerate ox pudding-stone,^ which may 
consist of pieces of one or of many different kinds of rock. 
A conglomerate, therefore, is simply gravel bound together 
by a cement. 
Argillaceous Rocks. —Clay, strictly speaking, is a mixture 
of silex or flint with a large proportion, usually about one 
fourth, of alumina, or argil; but in common language, any 
earth which possesses sufficient ductility, when kneaded up 
with water, to be fashioned like paste by the hand, or by 
the potter’s lathe, is called a clay; and such clays vary 
greatly in their composition, and are, in general, nothing 
more than mud derived from the decomposition or wearing 
down of rocks. The purest clay found in nature is porce¬ 
lain clay, or kaolin, which results from the decomposition of 
a rock composed of feldspar and quartz, and it is almost al¬ 
ways mixed with quartz. The kaolin of China consists of 
YriS parts of silex, 15*86 of alumine, 1*92 of lime, and 6*73 
of water ;* but other porcelain clays differ materially, that 
of Cornwall being composed, according to Boase, of nearly 
.equal parts of silica and alumine, with 1 per cenr. of magne- 
sia.f Shale has also the property, like clay, of becoming 
plastic in water: it is a more solid form of clay, or argilla¬ 
ceous matter, condensed by pressure. It always divides into 
laminae more or less regular. 
One general character of all argillaceous rocks is to give 
out a peculiar, earthy odor when breathed upon, which is 
a test of the presence of alumine, although it does not be¬ 
long to pure alumine, but, apparently, to the combination of 
that substance with oxide of iron. J 
Calcareous Rocks. —This division comprehends those rocks 
which, like chalk, are composed chiefly of lime and carbonic 
* W. Phillips, Mineralogy, p. 33. f Phil. Mag., vol. x., 1837. 
i See W. Phillips’s Mineralogy, ‘‘Alumine.” 
