although we often find them of the other sort, where little or
nothing of rock is to be seen. But often also we find the two cases
variously compounded. This is the source of the difficulty which occurs
in the reading of the external characters of mountains; and this is one
of the causes of irregularity in the form of mountains, by which there
is always some degree of uncertainty in our judgment from external
appearances.

We may form another distinction with regard to the structure of
mountains, a distinction which depends upon a particular cause, and
which will afford an explanation of some other appearance in the surface
of the earth.

Mountains in general may be considered as, being either on the one hand
associated, or on the other insulated; and this forms a distinction
which may be explained in the theory, and afford some ground for judging
of the internal structure from the external appearance.

The associated mountains are formed by the wearing down of the most
decayable, or softer places, by the collected waters of the surface;
consequently there is a certain similarity, or analogy, of the mountains
formed of the same materials, and thus associated. The highest of those
mountains should be near the center of the mass; but, in extensive
masses of this kind, there may also be more than one center. Nor are
all the associated mountains to be of one kind, however, to a certain
extent, similarity may be expected to prevail among them.

It must now be evident, that when we find mountains composed of very
different materials, such as, _e.g._ of granite, and of lime-stone or
marl, and when the shape of those mountains are similar, or formed upon
the same principle, such as, _e.g._ the pyramidal mountains of the Alps,
we are then to conclude, as has already been exemplified (chap. 9. page
306.) that those consolidated masses of this earth had been formed into
the pyramidal mountains in the same manner. We have there also shown
that this principle of formation is no other than the gradual decay of
the solid mass by gravity and the atmospheric influences. Consequently,
those pyramidal mountains, though composed of such different materials,
may, at a certain distance, where smaller characteristic distinctions
may not be perceivable, appear to be of the same kind; and this indeed
they truly are, so far as having their general shape formed upon the
same principle.

We come now to treat of insulated mountains. Here volca