ELEMENTARY FACTS. 
57 
CHAPTER V. 
ELEMENTARY FACTS AND PRINCIPLES OF GEOLOGY. 
Our Knowledge of the Internal Structure of the Earth very 
Limited. — Variety of Minerals and Rocks, arranged in a de- 
terminate Order. — Advantages of Geological Know^ledge. — 
Division of Rocks: Stratified, XJnstratified, Parallel, In- 
chned, Cropping out, Dip, Thickness, Outliers, Escarpment, 
Faults, Mineral Veins, Rock Dike, Clay Dike, Formations. — 
Illustrations. — How to Observe. — Valleys of Denudation, &c. 
Having considered the properties of matter, the 
chemical and mineralogical constitution of the 
earth's surface, and some points connected with 
its physical geography, we are now prepared to 
contemplate those general truths, obtained by obser- 
vation and experiment, concerning the arrangement 
of these materials, so far as they may be open to 
our inspection. We at once, however, perceive 
that our investigation must necessarily be confined 
to a comparatively small portion of the mass of the 
earth, namely, its superficial external crust, which 
the efibrts of man have hitherto failed to penetrate 
to a greater depth than about 3000 feet. To this 
we may add the elevation of the highest mountains, 
and then we find our direct knowledge is confined 
within the limits of about one eight hundredth part 
of the earth's semi-diameter ; a perpendicular dis- 
tance varying from five to eight or ten miles ; bear- 
ing a proportion to the mass of the earth less than 
that of a thin coat of varnish to an artificial globe 
of two feet diameter. One of the first things which 
strikes our observation, as we begin to notice the 
materials of which this crust is composed, is, that 
they do not form a simple homogeneous mass, con- 
