A Mine Dressed for Examination

A mine that has beendressed for examination may be described as a trap for the examiningengineer. The favorable features are accentuated, andthe unfavorable developments are concealed,—drifts are walledup, or are allowed, or encouraged, to cave, winzes are allowed to fillwith water, and workings through poor stretches of the depositare tightly timbered to hinder examination. In stoping operationsbarren material is removed up to the best showings, whichare left undisturbed, on the supposition that the engineer willassume that the material stoped was payable ore. Exploratorydrifts are frequently stopped in good ore; it is astonishing in howmany cases a man who is familiar with the local ore-shoots canstop his drift just short of running into barren ground.

Vigilance and a suspicions attitude are the engineer's -«afeguards against this kind of fraud. A method followed by the writer is almost sure to result in detection if trickery has been attempted. On going through the mine many questions are asked in regard to every point that suggests itself, and the answers are set down. The questions are repeated and answers noted from as many of the chief men on the ground as may be induced to answer them. After an interval the questions are repeated, but not in the same order, and the answers are again noted. Before leaving the property a third inquisition will yield the desired result. If the local representatives have been telling the truth, their answers will check up, but it is a very exceptional liar that can stick to a fabric of falsehoods three separate times with long intervals between.

Past Production.—A study of smelter or mill returns from past ore shipments in connection with the amount of exploration done is often instructive in the examination of irregular deposits. The average value of past ore shipments, of course, is no criterion of the grade of ore left in the mine, as the best ore is invariably extracted first.

Low-Grade Ores.—A slightiy explored property carrying lowgrade ore that may be expected just to pay its way should be explored, as higher-grade ore may be found. Any large deposit of even very low-grade material should be given at least a preliminary examination, as the constant improvement in metallurgical processes is steadily rendering payable ores of lower and lower grade. The wast« of 15 years ago is the ore of today, and the same advance may be reasonably expected in the future.

Large Versus Small Properties.—In general, a large body of low-grade ore is more likely to persist than a smaller bodj'' of high-grade ore. Small mines, unless containing shipping ore and requiring little equipment (and these are rarely purchasable at a reasonable price), are usually not profitable. The cost of all equipment must in the final analysis be charged against the tonnage of ore extracted, and one mine may jdeld a handsome profit where two mines each containing one-half the tonnage of the same grade may both net losses. Furthermore, a small property is in a poor strategic position to secure good smelting or freight rates.