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. The demand of a country like England can only be supplied by taking salmon from many different rivers of unequal productiveness, and the value must be sufficient to repay the cost of obtaining the fish from the least productive of these. All others, therefore, will if appropriated afford a rent equal to the value of their superiority. Much higher than this it cannot be, if there are salmon rivers accessible which from distance or inferior productiveness have not yet contributed to supply the market. If there are not, the value, doubtless, may rise to a scarcity rate, and the worst fisheries in use may then yield a considerable rent.
Both in the case of mines and of fisheries, the natural order of events is liable to be interrupted by the opening of a new mine, or a new fishery, of superior quality to some of those already in use. The first effect of such an incident is an increase of the supply; which of course lowers the value
