CO885-(21-23) — Page 487

PUBLIC RECORD OFFICE

Reference :-

wwwii C.O. 885

22 PUBLIC RECORD OFFICE, LONDON

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XIII.

SAND AND GRAVEL.

Specimens of sand may be packed in the same manner as clay, but care is required to prevent the contents of the packet from escaping. Samples of gravels will rarely be required, though a trained geologist may study them and make a selection from the stones they contain, which will afford valuable information as to the source of the materials of which they are composed.

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It will usually, however, be of advantage to "concentrate the heavier constituents of sand and gravel by washing or "panning," as the concentrates thus obtained afford impor- tant evidence of the mineral resources of the area from which they were derived and may sometimes contain valuable con- stituents.

For this purpose a circular iron pau about 34 inches deep, and 15 inches in diameter at the top and 8 inches at the bottom is employed. Some washing pans are provided with grooves or riftles on the sloping sides, but these are 'not absolutely necessary. In Brazil a wooden pan or "batea is used with good results.

The pan is nearly filled with the material to be washed, and then shaken vigorously to and fro in a horizontal position. This causes the larger stones to come to the top. They are picked out and examined to see if they present any points of interest, and if uot, are thrown away. The pan is then filled up with water and after being well shaken it is tilted so as to allow the sand to dribble slowly over the edge while the shaking is continued, fresh supplies of water being taken in from time to time. This pro- cess is carried out until only about two ounces of the sand remain. The greater part of the valueless siliceous and fels- pathic material will now have been removed and the remaining sand of the concentrate will contain all the heavier minerals, some of which might be lost if further concentration were attempted. In the neighbourhood of a stream the panning can he best carried out with the lower rim of the pan dipping under the water.

The concentrate should be dried and collected into a sheet of paper which should be carefully folded up and sealed. The exact locality and depth from which the sand or gravel was taken, the number of paus which were washed to obtain the con- centrate, and a distinguishing number or letter should be written on it. It should then be wrapped up in a second sheet of paper, also marked with the locality, and packed with similar specimens, so closely as to allow no movement, in a small box with a tightly fitting lid. Unwashed samples from the same localities should also be taken.

If the sand or gravel to be panned contains much clay, this interferes seriously, by its clogging effect, with the concentration and must first be removed by adding water and stirring with the hand, after which the water is carefully poured off with the sus pended clay. This operation must be repented until the water that is poured away is almost clear and the residue is practically

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free from clay. Here again the operation is much expedited if it be carried out under water, especially that of a gently flowing

stream.

The bent position of the body in panning is rather tiring, aud, in tropical countries, if the sun is at all high, some protection for the back is required. As a' general rule it is easy to instruct native workmen or servants in the operations of panning and the actual work may be left to them, subject, of course, to close super- vision.

XIV.

OIL, WATER, AND GAS.

If oil is found oozing from the ground, a hole a foot or two deep should be dug at the spot and allowed to fill with oil, and from this as clean a sample as posible should be taken, but it must be remembered that crude oil is usually dark and malodorous, very different from the refined material employed for illuminating purposes. A gallon should, if possible, be ob- tained to enable the oil to be properly examined. It may be transported in bottles, which should be washed out with oil from the same locality before they are filled. When each bottle is nearly full of oil a little water may be added, after which the bottle must be securely corked and sealed. If, as is usually the case, the oil is lighter than water, the bottle should be carried with the mouth vertically downwards. If the oil is heavier than the water the mouth should be kept upwards. In this way the water will prevent the escape of the volatile constituents which form the most valuable portion of the oil.

In

Samples of water may be taken either to ascertain whether it is suitable for driuking or for use in steam engines or com- mercial processes, or because it is believed to contain salts or other constituents of medicinal or economic importance. either case a gallon is required for a satisfactory analysis. Care should be taken that the water should be free from surface im- purities and for this purpose, in the case of a spring, u small basin should if necessary be excavated. The water may be stored in wine bottles, which should be first washed out with the same water. They should of course be carefully corked and sealed. In many cases it may be desirable to evaporate the water to dryness to save transport. The residue thus obtained should be placed in a bottle, which should be well corked, for the salts contained in mineral springs are sometimes hygroscopic. This dried residue should not be regarded as a sufficient substitute for a sample of the water in its natural condition, for chemical changes frequently occur in the course of evaporation, and even may differ

a sample which has been only partly evaporated

in important respects from the original water.

To collect a sample of gas which is escaping from the ground a bottle should be completely filled with water, corked, and in- verted into the orifice from which the gas is escuping. The cork