This system need not be considered in detail; the foundation of it is laid in such defiance of the principles of geometry and mechanics, that the architect, notwithstanding all the fertility of his invention, and all the resources of his genius, was never able to give any solidity to the structure. But it will be said, that we may take a part of the system, without venturing on the whole, and may suppose that the earth, or at least the external crust of it, has been fluid by fire, though we do not inquire into the cause of this fire, or into the manner in which it was produced. It is indeed true, that, when this is done, we have not the same sort of absurdity to encounter that we met with in the Neptunian system, and that the Volcanic theory does not, like it, come into direct collision with an axiom of geometry. There are, nevertheless, great objections to it; for though all the phenomena of the mineral kingdom attest a fluidity of igneous origin, yet it is a fluidity that was never more than partial; and though it has been over all the earth, has been over it in succession only. Besides, we are not entitled to assume the existence, and again the disappearance of such a great quantity of heat, without assigning some cause for the change. 435. Since, then, neither the hypothesis of the Neptunists or the Vulcanists, affords any good explanation of the figure of the earth, or such a one as can connect it with the other appearances in its natural history, it remains to inquire, whether the system that supposes a partial and successive fluidity, like Dr Hutton's, has any resource for explaining this great phenomenon. Of this subject Dr Hutton has not treated; and when I was first made acquainted with his system, it appeared to me a very serious objection to it, that it did not profess to give an explanation of so important a fact as the oblate figure of the earth. On considering the matter more closely, however, I found that there were principles contained in it from which a very satisfactory solution (and, I think, the only satisfactory solution) of that difficulty might be deduced. This solution I shall endeavour to explain, in as far, at least, as is necessary for the purpose of general illustration. It is laid down in Dr Hutton's theory, that the surface of the earth is perpetually changed by the _detritus_ of the land; and that from the materials thus afforded, new horizontal strata are perpetually formed at the bottom of the sea. If this be true, and if the alternations of decay and renovation have been often repeated, it is certain, that the figure of the earth, whatever it may have originally been, must be brought at length to coincide with the spheroid of equilibrium. 436. Here it is necessary to remark, that the expressions, _figure of the earth_, and _surface of the earth_ are each of them occasionally taken in two different senses.