DREDGING FOR GOLD.

Otago Witness, Issue 2318, 4 August 1898, Page 18

DREDGING FOR GOLD.

Gold-mining operations in the country lying west of the Rocky Mountains are just now being carried on by two very different systems. In the frozen North the pioneer is searching |for the rich placer deposits where the fragments of gold are so large and so thickly strewn that a single season's work with the miner's pan may bring- a fortune. In his feverish haste to grow rich the miuer pays no attention to the finer gold, which is allowed to run to waste in the tailings.

(Scientific American.)

In the historic goldfields of California, on the other hand, mining men are giving- increased attention to the recovery of gold from ton is very low. Improved machinery, capable of handling the auriferous material at ihe rate of thousands of tons per day, is being employed on gold-bearing deposits which hitherto it has been considered unprofitable lo work. The results have been very grati fying, and many discarded or neglected districts will how acquire a positive value.

The Hiscion Improved Rivei* Gold Dredge was designed by R. H. Postlethwaile, of San Francisco, California, patents for which are hold by the llisdon Iron and Locomotive iWorks, of that city. These dredges are the result of a process of evolution and many years of experiments by the designer and others in New Zealand, now recognised as the leading gold-dredging country of the world, from which country Mr Posllethwaito arrived in April 1897, for tho purpose of introducing and operating his dredger in this country.

One of those dredgers is now operating on the Yuba River, in California, and is lifting and washing over 93 cubic yards of gravel pur hour from a depth of 45ft, and extracting and Baring the gold therefrom, some of which is bo fine that it cannot be seen by the uaked eye, at a cost of 3c per yard. The dredger consists of two long pontoons, each 86ft long by 9ft beam. These are connected at the stern by a small pontoon 17ft long and sft wide, the bow being connected by a heavy overhung beam. This practically makes one boat '£6ft long and 23ft in width, with a well hole 51 1 wide running through the centre for some 75f1.

The dredger is fitted with a power winch with six drums, all being under tho control of one man. Four of these drums carry lines running from the four corners of the dredger, the other end 'of the linos being affixed to •"dead men" or backers on the beach. The fifth drum carries the head line. With thc.-ij five lines the dredger can bo made to rapidly take up any position necessary, one man handling her with the greatest ease and nicety, and with no loss of lime. The sixth winch barrel carries the ladder line, raising or lowering the ladder as required. A ladder 67ft long, built up as a heavy lattice girder, is hung at the stern end by a bar fixed across a heavy wooden framing. The lower end of the ladder carries a five-sided tumbler, and is suspended by blocks and tackle to a crossbeam. }'y means of a wire rope and blocUfi the winch fan raise or lower the bottom end as required. The top tumbler is carried by the limber framing some 3ft above the top end of the ladder. The continuous bucket chain comes up the top side of the ladder on rollers round the lop tumbler and back in a catenary curve to the lower tumbler. The top tumbler is driven through a ropo transmission and heavy gears by the engine.

a vertical compound condensing one, which also drives the pump and indicates 35 horsepowfr. The buckets discharge tbe material on to a { delivery plate, down which it shoots into a ' revolving screen or grizzly. The centrifugal pump, throwing 30Q0j»al per minute, supplies ' water lo a perforated pipe inside the screen. ' This water thoroughly washes the material, the finer wash dirt aad gold going through j holes in the pcreon and idling into a distributing box. From the distributing box it passes on to a set of gold-saving tables, lift wide, over which the wash dirt rims in a thin or shallow stream, and thence into a flume. Tho tables are covered with cocoa matting and expanded metal, a finer gold-paver than vvhic.i was never used. The stones and rocks pass through the screen down a &tone shoot, eitho direct into the river or, when working into a high face of gravel, on to a tailings elevator. i Broadly sneaking, with such a dredger as is

above described, any ground v\hk'h is not deeper than 60ft below water level, nor more than 20ft above, and which contains boulders of not more than, say, one ton we%ht, can be handled at from 3c to Sc per cubic yard. Tho ground need not be in a river, provided the ■seepage is sufficient to float the dredger tuid keep the water clean enough to wash the dirt with. The introduction of this dredger will revolutionise placer mining in this country, and will render valuable large tracts oi land heretofore, on account of their low grade condition, unworkable, and con&equently woilhle?s.