COMPOUND MINERALS, 
OR 
ROCKS. 
249. THERE exist considerable masses of minerals in 
a state of combination, or aggregation with each other. 
These constitute the rocks and soil of which the globe 
of the earth is composed ; and the study of them is 
called GEOLOGY. The opinions of learned men relative 
to their structure, and original formation, have produc- 
ed various systems denominated theories of the earth ; 
but, when we consider that the greatest depth beneath 
the surface to which the art and industry of man have 
been able to penetrate, does not exceed -3-5-0-0-0- part of 
the earth's diameter, we must confess that this is very 
insufficient to allow of any correct opinion being 
thereby formed concerning the structure of the whole. 
Modern geologists, for the more convenient arrange- 
ment of the compound minerals, have divided them 
into four classes, which they denominate primitive rocks, 
secondary rocks, alluvial depositions, and volcanic rocks. 
I. PRIMITIVE ROCKS. 
250. These are so calledfrom their being considered by 
geologists, to belong to the Jirst formed parts of the globe. 
Rocks of this description are of a nature extremely 
hard. They contain no vestiges whatever of animal 
or organic remains; and the substances of which they 
are composed are crystallized. They rise through other 
rocks at various elevations, in every quarter of the 
globe ; and never either alternate with, or rest upon 
rocks that enclose organic remains, though they are 
themselves frequently covered by such. 
The following are the principal kinds. 
