To explain the structure of mines, it should be observed that the internal parts of the earth, as far as they have been investigated, do not consist of any one uniform substance, but of various strata, or beds of substances, extremely different in their appearances, specific gravities and chemical qualities, one from another. Neither are these strata similar to each other, either in their nature or appearance, in different countries; insomuch that, even in the short extent of half a mile, sometimes, the strata will be found quite different in one from what they are in another place. As little are they the same either in depth or solidity. Innumerable cracks and fissures are found in all of them; and these again are so entirely different in size and shape, that it is impossible to form any inference from what may have been met with, relative to that which remains to be explored. In Cornwall, the most common opinion entertained by the miners, is, that crude and immature minerals nourish and feed the ores with which they are intermixed in the mines; and that the minerals themselves will, in process of time, be converted into ores productive of those metals to which they have the nearest affinity, and with which they are most closely intermingled. And a distinguished professor, who is familiar alike with geology, chemistry and mineralogy, after visiting the mining districts of California, has given it as his opinion, that gold is constantly being formed there, by some powerful agency of nature which is still and steadily at work. And as a somewhat kindred view, Mr. Price, in his mineralogy of Cornwall, thinks it is most reasonable to conclude, that metals were made and planted in veins, at, or very soon after, the creation of the world; but that, in common with all other matter, they are subject to a degree of fluctuation, approaching to, or receding from, their ultimate degree of perfection, either quicker or slower, as they are of greater or less solid and durable frame and constitution. He supposes in every metal a peculiar magnetism, and an approximation of particles of the same specific nature, by which its component principles are drawn and united together; more particularly the matters left by the decomposition of the waters passing through the contiguous earths or strata, and deposited in their proper nidus or receptacle, until, by the accretion of more or less of its homogeneous particles, the metallic vein may be denominated either rich or barren. DIAMOND MINES.