Agricultural productions are not the only commodities which have several different costs of production at once, and which, in consequence of that difference, and in proportion to it, afford a rent. Mines are also an instance. Almost all kinds of raw material extracted from the interior of the earth—metals, coals, precious stones, etc.—are obtained from mines differing considerably in fertility—that is, yielding very different quantities of the product to the same quantity of labor and capital. There are, perhaps, cases in which it is impossible to extract from a particular vein, in a given time, more than a certain quantity of ore, because there is only a limited surface of the vein exposed, on which more than a certain number of laborers can not be simultaneously employed. But this is not true of all mines. In collieries, for example, some other cause of limitation must be sought for. In some instances the owners limit the quantity raised, in order not too rapidly to exhaust the mine; in others there are said to be combinations of owners, to keep up a monopoly price by limiting the production. Whatever be the causes, it is a fact that mines of different degrees of richness are in operation, and since the value of the produce must be proportional to the cost of production at the worst mine (fertility and situation taken together), it is more than proportional to that of the best. All mines superior in produce to the worst actually worked will yield, therefore, a rent equal to the excess. They may yield more; and the worst mine may itself yield a rent. Mines being comparatively few, their qualities do not graduate gently into one another, as the qualities of land do; and the demand may be such as to keep the value of the produce considerably above the cost of production at the worst mine now worked, without being sufficient to bring into operation a still worse. During the interval, the produce is really at a scarcity value. Fisheries are another example. Fisheries in the open sea are not appropriated, but fisheries in lakes or rivers almost always are so, and likewise oyster-beds or other particular fishing-grounds on coasts. We may take salmon-fisheries as an example of the whole class. Some rivers are far more productive in salmon than others. None, however, without being exhausted, can supply more than a very limited demand. All others, therefore, will, if appropriated, afford a rent equal to the value of their superiority.