This theoretical question of our author is so properly connected with the natural history which he has here given us, that it is not difficult to resolve it in the most satisfactory manner.

Here is an enormous mass of granite, the origin of which we are not now inquiring after, but the causes of its present form.  The internal part of this granite subsists in a state of the most perfect solidity; the external again is evidently in a decaying state.  This is a fact which we learn from the nature of feldspar, of which granite is in part composed; this crystallised substance is every where decomposed, where long exposed to the atmosphere.  But it is not this gradual decay of the mass of granite perishing equably from its external surface, and resolved into some of its component parts, that we are here to consider; it is only mentioned to show that the mass of granite is subject to decay, when exposed to the influence of the atmosphere, like every other compound mineral body, and to lose that perfect solidity which we find in the centre of the mass.

We find the granite masses not only subject to decay from the external surface, by the decomposition of the feltspar, or the dissolution of its constituent parts, but also liable to be separated into blocks of different degrees of regularity, commonly rectangular or approaching to the rhombic shape.  This is the consequence, either of larger veins and fissures, filled with matter which is still more dissolvable than is the substance of the granite, or else by imperceptible crevices or cutters, into which the atmospheric influences gradually insinuate, and form at last a visible separation.

In examining the tops of granite mountains, or where this rock is exposed to the weather, we may perceive those two species of decay proceeding together.  The external surface of the stone, where there is a sufficient mixture of feltspar, is separating into grains which form a species of sand, being nothing but the particles of granite separating by means of the decaying sparry part.  But a similar progress may be observed, from the external surface penetrating in lines the mass of solid rock, and dividing that mass into the rectangular blocks into which those exposed places are gradually resolved.

Now the tops of all those mountains are formed into an assemblage of pyramids, declining in height from the central pyramid; and all those pyramids are again in like manner subdivided into lesser pyramids.  But the smallest of those pyramids are no other than the rectangular blocks into which those granite masses always separate by the influence of the atmosphere.

It will now be evident, that those mountains, thus resolving into separate blocks, must acquire this series of pyramidal constructions; for, in every particular mass of mountain, there must be a central part, from which the separated blocks cannot be removed, while those around, or towards the sides, are detached by the swelling water upon freezing, and separated from the more central masses which are thus the latest of being removed.