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 volcanos must be mentioned as a cause.  By means of a volcano, a mountain may be raised in a plain, and a volcanic mountain might even rise out of the sea.  The formation of this species of mountain requires not the wearing operations of the earth which we have been considering as the modifier of our alpine regions.  This volcanic mountain has a conical shape, perhaps more from the manner of its formation which is accretion, than from the wasting of the surface of the earth.  It is not, however, of this particular specie of mountain that I mean to treat, having had no opportunity of examining any of that species.

The genus of mountain which we are now considering, is that of the eruptive kind.  But there is much of this eruptive matter in the bowels of the earth, which, so far as we know, never has produced a volcano.  It is to this species of eruption that I am now to attribute the formation of many insulated mountains, which rise in what may be termed low countries, in opposition to the highlands or alpine situations.  Such is Wrekin in Shropshire, which some people have supposed to have been a volcano.  Such are the hundred little mountains in the lowlands of this country of Scotland, where those insulated hills are often called by the general term Law; as, for example, North Berwick Law.