In explaining those appearances of degraded mountains variously shaped, the fact we are now to reason upon is this; first, that in the consolidated earth we find great inequality in the resisting powers of the various consolidated bodies, both from the different degrees of consolidation which had taken place among them, and the different degrees of solubility which is found in the consolidated substances; and, secondly, that we find great diversity in the size, form, and positions of those most durable bodies which, by resisting longer the effects of the wearing operations of the surface, must determine the shape of the remaining mass. Now so far as every particular shape upon the surface of this earth is found to correspond to the effect of those two causes, the theory which gave those principles must be confirmed in the examination of the earth; and so far as the theory is admitted to be just, we have principles for the explanation of every appearance of that kind, whether from the forming or destroying operations of this earth, there being no part upon the surface of this earth in which the effect of both those causes must not more or less appear.
But though the effects of those two causes be evident in the conformation of every mountainous region, it is not always easy to analyse those effects so as to see the efficient cause. Without sections of mountains their internal structure cannot be perceived, if the surface which we see be covered with soil as is generally the case. It is true, indeed, that the solid bodies often partially appear through that covering of soil, and so far discover to us what is to be found within; but as those solid parts are often in disorder, we cannot, from a small portion, always judge of the generality. Besides, the solid parts of mountains is often a compound thing, composed both of stratified and injected bodies; it is therefore most precarious, from a portion which is seen, to form a judgment of a whole mass which is unexplored. Nevertheless, knowing the principles observed by nature both in the construction and degradation of mountains, and cautiously inferring nothing farther than the data will admit of, some conclusion may be formed, in reasoning from what is known to what is still unknown.
It is with this view that we are now to consider the general forms of mountains, such as they appear to us at a certain distance, when we have not the opportunity of examining them in a more perfect manner. For, though we may not thus learn always to understand that which is thus examined, we shall learn, what is still more interesting, viz. that those mountains have been formed in the natural operations of the earth, and according to physical rules that may be investigated.
We are to distinguish mountains as being either on the one hand soft and smooth, or on the other hand as hard and rocky. If we can understand those two great divisions by themselves, we shall find it easy to explain the more complex cases, where these two general appearances partially prevail. Let us therefore examine this general division which we have made with regard to the external character of mountains.
The soft and smooth mountains are generally formed of the schisti, when there is any considerable extent of such alpine or mountainous region. The substance is sufficiently durable to form a mountain, or sufficiently strong, in its natural state, to resist the greatest torrent of water; at the same time this fissible substance generally decays so completely, when exposed to the atmosphere, as to leave no salient rock exposed by which to characterise the mountain.
Of this kind are the schisti of Wales, of Cumberland, of the isle of Man, and of the south of Scotland. I do not say absolutely, that there is no other kind of material, besides the schisti which gives this species of mountain, but only that this is generally the case in alpine situations. It may be also formed of any other substance which has solidity enough to remain in the form of mountains, and at same time not enough to form salient rocks. Such, for example, is the chalk hills of the Isle of Wight and south of England. But these are generally hills of an inferior height compared with our alpine schisti, and hardly deserve the term of mountain.
This material of our smooth green mountains may be termed an argillaceous schistus; it has generally calcareous veins, and is often fibrous in its structure resembling wood, instead of being slatey, which it is in general. There is however another species of schistus, forming also the same sort of mountain; it is the micaceous quartzy schistus of the north of Scotland. Now it must be evident that the character of those mountains arises from there being no part of those schisti that resists the influence of the atmosphere, in exfoliating and breaking into soil; and this soil is doubtless of different qualities, according to the nature of those schisti from which the soil is formed.
Such mountains are necessarily composed of rounded masses, and not formed of angular shapes. They are covered with soil, which is more or less either stoney or tender, sterile or fertile, according to the materials which produce that soil. The fertile mountains are green and covered with grass; the sterile mountains again are black, or covered with heath in our climates.
Thus we have a general character of smooth and rounded mountains; and also a distinction in that general character from the produce of the soil indicating the nature of the solid materials, as containing, either on the one hand calcareous and argillaceous substances, or, on the other, as only containing those that are micaceous and siliceous.
With regard again to the other species of mountain, which we have termed rocky, we must make a subdistinction of those which are regular, and those in which there is no regularity to be perceived. It must be plain that it is only of those which have regularity that we can form a theory. It is this, that the regularity in the shape of those mountains arises from the rock of the mountain being either on the one hand an uniform solid mass, or on the other hand a stratified mass, or one formed upon some regular principle distinguishable in the shape. In the first of these, we have a conical or pyramidal shape, arising from the gradual decay of the rock exposed to the destructive causes of the surface, as already explained in this chapter. In the second, again, we find the original structure of the mass influencing the present shape in conjunction with the destructive causes, by which a certain regularity may be observed. Now, this original shape is no other than that of beds or strata of solid resisting rock, which may be regularly disposed in a mountain, either horizontally, vertically, or in an inclined position; and those solid beds may then affect the shape of the mountain in some regular or distinguishable manner, besides the other parts of its shape which it acquires upon the principle of decay.