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.

In distinguishing, at a distance, those regular causes in the form of mountains, we may not be able to tell, with certainty, what the substance is of which the mountain is composed; yet, with regard to the internal structure of that part of the earth, a person of knowledge and experience in the subject may form a judgment in which, for coming at truth, there is more than accident; there is even often more than probable conjecture.  Thus, a horizontal bed of rock forms a table mountain, or such as M. Bouguer found in the valley of the Madelena.  An inclined rock of this kind forms a mountain sloping on the one side, and having a precipice upon the upper part of the other side, with a slope of fallen earth at the bottom; such as the ridges observed by M. de Saussure from the top of the Cramont, having precipices upon one side, which also had a respect to certain central points, an observation which draws to more than the simple structure of the mountain.  Were it vertical, again, it would form a rocky ridge extended in length, and having its sides equally sloped, so far as the other circumstances of the place would permit.

Therefore, whether we suppose the mountains formed of a rock in mass, or in that of regular beds, this must have an influence in the form of this decaying surface of the earth, and may be distinguished in the shape of mountains.  It is but rarely that we find mountains formed altogether of rock, 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.