Now, what cause can have determined the column of water, which rested on the base at present occupied by this rock, to deposit nothing but the materials of whinstone, while the water on the south, west, and north, was depositing the materials of arenaceous and marly strata? Wherefore, within this small space, was the precipitate every where _chemical_, to use the language of Werner, while close to it, on either side, it was entirely _mechanical?_ Why is there, in this case, no gradation? and why is a mere mathematical line the boundary between regions where such different laws have prevailed? Whence also, we may ask, has the basaltic deposit been abruptly terminated toward the west, so as to produce the steep face which has just been mentioned? The operation of currents, or of any motion that can take place in a fluid, will furnish no explanation whatever of these phenomena; yet they are phenomena far from being peculiar to a single hill; they are among the most general and characteristic appearances in the natural history of whinstone mountains; and a geological theory which does not account for them, is hardly entitled to any consideration. 257. The basaltic rock, just described, is also covered, at least partly, with strata perfectly similar to those that lie under it. Now, it appears altogether unaccountable, that after the water had done depositing the materials of the whin on the spot in question, the former order was so quickly resumed, and a deposition of sand, and of the other materials of the strata, took place just as before. All this is quite unintelligible; and the principles of the Neptunian system seem here to stand as much in need of explanation, as any of the appearances which they are intended to account for. 258. The unequal thickness, and great irregularity in the surface of the whinstone mass, here treated of, and of many rocks of the same kind, is also a great objection to the notion of their aqueous formation. This seems to have been perceived by Werner, in the instance of the rocks formerly mentioned; and he endeavours to explain it, by supposing, that much of these rocks has been destroyed by waste and decomposition, so that an irregularity of their surface, and want of correspondence has been given to them, which they did not originally possess. In the instance of Salisbury _Craig_, however, we have a proof, that the great irregularity of surface, and the inequality of thickness, do not always arise from these causes. The thinnest part of that rock, toward its northern extremity, is still covered by the strata in their natural place, and has been perfectly defended by them from every sort of wearing and decay. The cuneiform shape, therefore, which this rock takes at its extremities, and the great difference of its thickness at them and in the middle, is a part of its original constitution, and can be attributed to nothing casual, or subsequent to its consolidation.