Full text: Vol. IV. (4)

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,
	        
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