The Elements of economic geology

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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

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Document type:: Monograph

Structure type:: Chapter

Title:: Part II. Ore deposits

Collection:: Economics Books

1” ECONOMIC GEOLOGY washing in water. The conclusive evidence against the growth of nuggets in the gravels is their distribution. No nuggets are found in goldfields where the gold in the lodes is in minute particles; or they may be found, e.g. in the Rand in South Africa, only where coarse gold has been formed in secondary quartz-veins. In Ballarat East, as shown in Fig. 11, the nuggets were found along a line where nuggety patches occur in the lodes, These patches are exposed on the hillside ; pebbles washed down the slope, knocked out the brittle quartz and hammered the patch into a nugget. This origin is con- sistent with the investigations on nuggets of A. Liversidge (F.R. Soe. New South Wales, xxvii, 1893, p. 343; xl, 1906, p. 161). The mining of Ballarat East was dependent on some clue to the arrangement of these nuggety patches. One having been found where a quartz-vein met g vertical brown line, this line was followed and led to other patches of gold, where it met a quartz-vein. The ine was therefore called the in- dicator. Several of these indica- ‘ors have been found in the slates at Ballarat East, and have been traced for eight miles along the field. They vary from about one- sixteenth to a quarter of an inch in thickness; the main indicator sometimes divides into three layers which have a tota] thick- ness of half an inch. Their thickness and their dark colour, below the oxidized zone, is expressed by the names of the Pencil Mark and the Telegraph Line given to two of them, The microscopic structure of the indicators shows that most of them consist of bands of chlorite developed in the slate along planes of slipping (Fig. 12). The indicators in places occur along the cleavage planes, but often cross them. An

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