The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

Usage license:: Get license information via the feedback formular.

Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part II. Ore deposits

Collection:: Economics Books

34 ECONOMIC GEOLOGY called the * Father of Dredging,” proposed the use of the harbour bucket dredge, and one was tried on the Otago River in 1886. It was not a financial success but showed that the process was practicable, and specially designed bucket dredges proved efficient and extraordinarily economical. It is claimed (E. B. Wilson, Hydraulic Mining, 1898, p. 100) that ground less than 60 feet below or 20 feet above water- level, which does not contain boulders more than a ton in weight, should be handled by dredges at 13d. to 23d. a cubic yard, though the cost is often 5d. a cubic yard. Wilson (ibid., p. 106) states that a dredge may pay on a re- covery of £ of a grain of gold to the ton of gravel, and some have paid dividends in Victoria with material of that grade. A dredge may haul from a river a cubic yard of earth, wash it, separate its gold, and yield a profit if it contains a penny- worth of gold. In some rocky river beds the gold lies in the depressions and a bucket dredge can only recover it by breaking off the projections unless the river bed has been blasted, so that the material can be scooped out. Such places can be worked by the suction dredge, which by a stream of water sucks up a pipe all loose material and gold dust on the river bed. Suction dredges are also used on river-side flats; the dredge is built in an excavation; it works forward, depositing the ground washed from the front of the pit behind it; it is floated forward to a new position by flooding the excavation, and thus gradually works its way through the whole alluvial plain. The coarse boulders should be deposited at the bottom and the fine material on the top, so that the ground may be left in better condition for agriculture than before the dredge began its work. Deep Leaps—The river placers first worked lay on the beds of rivers or the floors or sides of valleys and were known as ‘leads.” Some exceptionally rich deposits were due to a recent valley having been cut through the deposits of an older valley, with the reconcentration of its gold. Thus (at XX in Fig. 17) the gravel was especially rich because that of an ancient river had been rewashed and the gold further concentrated. The continuation of the old river was found under the hills of sand and clay which have filled its valley, and is a buried lead or * deep lead.”

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