AUSTRALIAN MINING-FIELDS 



129 



At the end of the 'eighties the field was surveyed by Mr. 

 E. J. Dunn, 1 on behalf of the Victorian Geological Survey. His 

 report showed that the saddle reefs were characteristic of the 

 whole field. The slates and sandstones which form the country 

 rocks have been contorted into many parallel folds ; and the 

 quartz reefs have been deposited in the spaces left beneath a 

 hard bed, along the summits of the anticlinals. The reefs are 

 not always exactly along the bedding planes, for during the 

 bending of a band of slates, cracks will be formed obliquely 

 across the bedding, in a flatter curve than that of the main arch 

 (as in fig. 3, s.R. 3 ). 



Now at Bendigo the Ordovician rocks are a thick series of 



ir. 



Fig. 4. — False Saddle Reefs formed of an interbedded reef (J.r) and 

 a spur (s) along a joint plane. j j. Joint planes. 



alternating, graptolitic slates and sandstones, in which the same 

 sequence of deposition frequently recurs, owing to the repetition 

 of the same geographical conditions in the old Ordovician Sea. 

 Hence a line sunk downward along the axial plane of an anti- 

 clinal, the vertical line in fig. 3 will meet many positions where 

 interspaces occurred beneath sandstone arches ; and in all such 

 positions it is natural to find that saddle reefs have been formed. 

 Shoots of gold occur at intervals along these reefs. 



The discovery of the origin of the quartz reefs gave the clue 

 to their distribution. After the exhaustion of one saddle reef, 

 it is only necessary to sink the mine shaft downward to the 

 next similar position, and there another saddle reef is found. 



1 E. J. Dunn, Reports on the Bendigo Goldfield, No. 1, 1892 (reprinted 1896), 

 20 pp. and plans ; No. 2, 1896, 42 pp. with plates and plans. 



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