Thus, in examining geological and mineralogical theories, I am laid under the disagreeable necessity of pointing out the errors of physical principles which are assumed, the prejudices of theoretical opinions which have been received, and the misconceived notions which philosophers entertain with regard to the system of nature, in which may be perceived no ineffectual operation, nor any destructive intention, but the wise and benevolent purpose of preserving the present order of this world. But, though thus misled with regard to the cause of things, naturalists are every where making interesting observations in the mineral kingdom, I shall therefore avail myself of that instructive information, for the confirmation of my theory. It may now be proper to consider what must be required, in order to have a geological and mineral theory established upon scientific principles, or on such grounds as must give conviction to those who will examine the subject; for, unless we may clearly see that there are means for attaining that desirable end, few philosophers will be persuaded to pursue this branch of knowledge. A theory is nothing but the generalization of particular facts; and, in a theory of the earth, those facts must be taken from the observations of natural history. Nature is considered as absolutely true; no error or contradiction can be found in nature. For, if such contradiction were truly found, if the stone, for example, which fell to day were to rise again to-morrow, there would be an end of natural philosophy, our principles would fail, and we would no longer investigate the rules of nature from our observations. Every natural appearance, therefore, which is explained, _i.e._ which is made to come into the order of things that happen, must so far confirm the theory to which it then belongs. But is it necessary, that every particular appearance, among minerals, should be thus explained in a general theory of the earth? And, is any appearance, which is not explained by it, to be considered as sufficient to discredit or confute a theory which corresponded with every other appearance? Here is a question which it would require some accuracy to resolve.