Those other parts of the produce of land, however, which afterwards afford rent, do not afford it always. Even in improved and cultivated countries, the demand for them is not always such as to afford a greater price than what is sufficient to pay the labour, and replace, together with it ordinary profits, the stock which must be employed in bringing them to market. Whether it is or is not such depends upon different circumstances.

Whether a coal-mine, for example, can afford any rent depends partly upon its fertility, and partly upon its situation.

A mine of any kind may be said to be either fertile or barren, according as the quantity of mineral which can be brought from it by a certain quantity of labour is greater or less than what can be brought by an equal quantity from the greater part of other mines of the same kind.