Class I.
Order III.
Arrange
ment.
Composi
tion.
ACGREGATES.
Aggregates may be comprehended under four divi
sions: 1. Mixtures of earths ; 2. Amorphous fragments
of stones agglutinated together ; 3. Crystallized stones,
either agglutinated together or with amorphous stones;
4. Aggregates formed by fire. It will be exceedingly
convenient to treat each of these separately. I shall
therefore divide this order into four Sections.
SECT. I.
ACGREGATÉS OF EARTHS.
IHE most common earthy aggregates may be com
prehended under the following genera:
1. Clay,
3. Marl,
2. Colorific earths,
4 Mould.
GENUS I. CLAY.
Clay is a mixture of alumina and silica in various
proportions. The alumina is in the state of an impalpable
powder; but the silica is almost always in small grains,
large enough to be distinguished by the eye. Clay,
therefore, exhibits the character of alumina, and not of
silica, even when this last ingredient predominates. The
particles of silica are already combined with each other;
and they have so strong an affinity for each other, that
few bodies can separate them; whereas the alumina, not
being combined, readily displays the characters which
distinguish it from other bodies. Besides alumina and
silica, clay often contains carbonat of lime, of magnesia,