of
such lower groups as those of Cumberland, Scotland, or South Italy.

All mountains whatsoever, not composed of the granite or gneiss rocks
described in the preceding chapter, nor volcanic, (these latter being
comparatively rare,) are composed of _beds_, not of homogeneous, heaped
materials, but of accumulated layers, whether of rock or soil. It may be
slate, sandstone, limestone, gravel, or clay; but whatever the
substance, it is laid in layers, not in a mass. These layers are
scarcely ever horizontal, and may slope to any degree, often occurring
vertical, the boldness of the hill outline commonly depending in a great
degree on their inclination. In consequence of this division into beds,
every mountain will have two great sets of lines more or less prevailing
in its contours--one indicative of the surfaces of the beds, where they
come out from under each other--and the other indicative of the
extremities or edges of the beds, where their continuity has been
interrupted. And these two great sets of lines will commonly be at right
angles with each other, or nearly so. If the surface of the bed approach
a horizontal line, its termination will approach the vertical, and this
is the most usual and ordinary way in which a precipice is produced.

Sec. 2. Farther division of these beds by joints.

Farther, in almost all rocks there is a third division of substance,
which gives to their beds a tendency to split transversely in some
directions rather than others, giving rise to what geologists call
"joints," and throwing the whole rock into blocks more or less
rhomboidal; so that the beds are not terminated by torn or ragged edges,
but by faces comparatively smooth and even, usually inclined to each
other at some definite angle. The whole arrangement may be tolerably
represented by the bricks of a wall, whose tiers may be considered as
strata, and whose sides and extremities will represent the joints by
which those strata are divided, varying, however, their direction in
different rocks, and in the same rock under differing circumstances.

Sec. 3. And by lines of lamination.

Finally, in the slates, grauwackes, and some calcareous beds, in the
greater number, indeed, of _mountain_ rocks, we find another most
conspicuous feature of general structure--the lines of lamination, which
divide the whole rock into an infinite number of delicate plates or
layers, sometimes parallel to the direction or "strike" of the strata,