tion into small folio) is, in these rocks, not always, nor
even often indicative of the true direction of their larger beds. It is
not, however, necessary for the reader to enter into questions of such
complicated nature as those which belong to the study of slaty cleavage;
and only a few points, which I could not pass over, are noted in the
Appendix; but it is necessary to observe here, that all rocks, however
constituted, or however disposed, have certain ways of breaking in one
direction rather than another, and separating themselves into blocks by
means of smooth cracks or fissures, technically called joints, which
often influence their forms more than either the position of their beds,
or their slaty lamination; and always are conspicuous in their weathered
masses. Of these, however, as it would be wearisome to enter into more
detail at present, I rather choose to speak incidentally, as we meet
with examples of their results in the scenery we have to study more
particularly.

FOOTNOTES

[48] Compare the close of Sec. 11, Chap. III. Vol. III., and, here,
Chap. III. Sec. 23.

CHAPTER XI.

OF THE MATERIALS OF MOUNTAINS:--FOURTHLY, COMPACT COHERENTS.

Sec. 1. This group of rocks, the last we have to examine, is, as far as
respects geographical extent and usefulness to the human race, more
important than any of the preceding ones. It forms the greater part of
all low hills and uplands throughout the world, and supplies the most
valuable materials for building and sculpture, being distinguished from
the group of the slaty coherents by its incapability of being separated
into thin sheets. All the rocks belonging to the group break
irregularly, like loaf sugar or dried clay. Some of them are composed of
hardened calcareous matter, and are known as limestone; others are
merely hardened sand, and are called freestone or sandstone; and others,
appearing to consist of dry mud or clay, are of less general importance,
and receive different names in different localities.

Sec. 2. Among these rocks, the foremost position is, of course, occupied by
the great group of the marbles, of which the substance appears to have
been prepared expressly in order to afford to human art a perfect means
of carrying out its purposes. They are of exactly the necessary
hardness,--neither so soft as to be incapable of maintaining themselves
in delicate forms, nor so hard as always to require a blow to give
effect to the sculpto