The profit per ton for a given deposit may have been extremely variable in the past, making it difficult to determine whether the highest or lowest figure should be projected into the future or whether some average should be taken; and if an average, whether the time covered by the average should be long or short. For a small, short-lived deposit obviously the most recent conditions would be taken into account in estimating future profits. For a long-lived property there would be more tendency to consider the long-time average vicissitudes, as reflected in the average profits of the past. For some mineral commodities there are cycles of prices, costs, and profits, of more or less definite length, established during the long past history of the industry; and in such cases it is desirable in calculating averages to use a period covering one or more of these cycles, rather than some shorter or longer period. For many minerals, however, these cycles have been too irregular to afford a sound basis for future estimates. If the experience of the property itself is too short to afford a sufficient foundation for forecasting profits, or if there has been no previous work on the property, then it is necessary to use averages based on other properties or other districts; or if there are none strictly comparable, to build up a hypothetical figure from various estimated costs of labor, supplies, and transportation, selling prices, etc. In the estimate of the profit factor, the geologist is not primarily concerned. In estimating the total reserves in a mine, geological considerations nearly always play a large part. An ore body may in some few cases be completely blocked out by underground work or drilling, eliminating the necessity for inferring conditions beyond those actually seen; but in the huge majority of mineral deposits the reserves are not so definitely known, and it becomes necessary for the geologist, through knowledge of similar occurrences, through study of the structural features of the deposit, its origin, and its history, to arrive at some sort of an estimate of reserves. In estimating the life of a mineral deposit it is necessary to start with the figure of total reserves, and from a study of conditions of mining and of markets to estimate the number of years necessary to exhaust the deposit. This is a more nearly commercial phase of the problem, in which the geologist takes only part of the responsibility. Perhaps more estimates of value have gone wrong because of misjudgment of this factor than for any other cause. If the physical conditions are satisfactory, it is easy to assume a rate of extraction and life based on hope, which experience will not substantiate.