Placing the Samples

The interval between samples depends upon the regularity with which the valuable minerals are distributed. One extreme might be considered an absolute uniform mass, of whicli one sample would suffice, anc the other extreme, a segregation of all the valuable minera' into a single mass; it is therefore apparent that the propei interval between samples wil differ with each exposure sampled.

In general, a 20-ft. interva will suffice for a large ore-shool of uniform grade, a 10-ft. interval in average cases M'here many samples are to be taker from the same ore-shoot, and 5-ft. or lesser intervals wheri the ore is spotty. It is usually advisable to start with 20-ft a property where no data are available, and resample at 10-ft. intervals, and perhaps again at 5-ft. intervals, where the results from the first series indicate that such a course is advisable.

In sampling a wide vein or deposit it is best to divide the width into sections and to sample them separately, in order tt determine the distribution of the values. These widths may be taken over even multiples of the total width if the deposit pre-sents a uniform appearance. If the vein or deposit presents a variegated or banded appearance, however, the several bands or zones should be sampled separately.

Where a section of ore is irregularly exposed, as is commonly the case with a vein in the roof of a drift, the sample must be cut deeper over the part that is at right angles to the vein than where the face is slanting, in order not to get an undue proportion from the slanting exposure.

The Size of Samples to be Taken.—The size of a sample should be limited to the least amount that will yield a true average of the exposure sampled. A few large samples are of little value as compared with many smaller samples, if the latter be well taken. Car-load shipments, ton-samples, and shooting down large samples are obsolete methods, as a bunch of rich ore is capable of salting the whole sample, and any sample that is too large to be sealed in a sack and properly protected against salting is a source of danger. Small mill runs are not satisfactory on any but very high-grade ores, as the clean-up will depend largely upon whether the plates are scraped clean or whether they are allowed to absorb amalgam.

The more uniform the ore the smaller may be the samples; where the ore is spotty, the samples should be large, as is also the case where the ore is loose and breaks irregularly, or is alternately hard and soft.

In cutting a sample the rich spots should be avoided if they are few in number; if there are many rich spots, che groove should include everjhing along its line.

It is a very difficult matter to sample correctly a spotty ore. In the case of an ore that consists of barren or nearly barren quartz carrying free gold, the average cannot well be determined by sampling; a majority of the samples of such an ore will be very low, or blanks, and a few will be very high; the average obtained will be more a matter of luck than a basis for an accurate estimate ; a large mill run is the solution of this problem.