r, and their age is also
for the most part approximately indicated by their degrees of hardness,
and crystalline aspect. It does, indeed, sometimes happen that a soft
and slimy clay will pass into a rock like Aberdeen granite by
transitions so subtle that no point of separation can be determined; and
it very often happens that rocks like Aberdeen granite are of more
recent formation than certain beds of sandstone and limestone. But, in
spite of all these uncertainties and exceptions, I believe that unless
actual pains be taken to efface from the mind its natural impressions,
the idea of three great classes of rocks and earth will maintain its
ground in the thoughts of the general observer; that whether he desire
it or not, he will find himself throwing the soft and loose clays and
sands together under one head; placing the hard rocks, of a dull,
compact, homogeneous substance, under another head; and the hardest
rocks, of a crystalline, glittering, and various substance, under a
third head; and having done this, he will also find that, with certain
easily admissible exceptions, these three classes of rocks are, in every
district which he examines, of three different ages; that the softest
are the youngest, the hard and homogeneous ones are older, and the
crystalline are the oldest; and he will, perhaps, in the end, find it a
somewhat inconvenient piece of respect to the complexity and accuracy of
modern geological science, if he refuse to the three classes, thus
defined in his imagination, their ancient title of Tertiary, Secondary,
and Primary.

Sec. 6. But however this may be, there is one lesson evidently intended to
be taught by the different characters of these rocks, which we must not
allow to escape us. We have to observe, first, the state of perfect
powerlessness, and loss of all beauty, exhibited in those beds of earth
in which the separated pieces or particles are entirely independent of
each other, more especially in the gravel whose pebbles have all been
_rolled into one shape_: secondly, the greater degree of permanence,
power, and beauty possessed by the rocks whose component atoms have some
affection and attraction for each other, though all of one kind; and
lastly, the utmost form and highest beauty of the rocks in which the
several atoms have all _different shapes_, _characters_, and _offices_;
but are inseparably united by some fiery process which has purified them
all.

It can hardly be necessary to