396 F. P. Mennell — Wood's Point Dijke, Victoria, Australia. 



cordierite gneiss to accouat for the occun-ence, but we have plenty of 

 aluminous material in the adjacent shales, and, as Teall has justly 

 pointed out,^ there is no reason why the mineral should actually be 

 present in the absorbed rook. None of the other minerals which are 

 genetically connected with cordierite and almost constantly accompany 

 it are present in the numerous slices I have examined, unless some 

 small brilliantly polarizing crystals are to be referred to sillimanite. 

 No separate determination of the alumina has been made, but the 

 combined alumina and ferric oxide come tol9'8 per cent., an extremely 

 high figure in so acid a rock, leaving less than 3 per cent, unaccounted 

 for, after taking the silica into consideration. The iron certainly 

 cannot be present in large quantities, ilmenite, pyrites, etc., being 

 entirely absent from this part of the rock, while the ferro-magnesian 

 minerals (chloritized hornblende and granular augite) are very 

 sparingly distributed. 



The intrusion of such dykes as the one described is evidently 

 related to the formation of the associated auriferous veins. In the 

 case of the Victorian Silurian deposits the reefs are almost invariably 

 connected with intrusive rocks, while those in the Ordovician strata 

 usually occur at or near the contact with great plutonic masses. It 

 seems highly probable, as Mr. Howitt has suggested,^ that on 

 following down a dyke formation it would be found to end as 

 a contact deposit on reaching its plutonic source. At Wood's Point 

 the reefs are nearly horizontal, traversing the dyke and the shales in 

 the immediate vicinity, the junction usually marking the occurrence 

 of rich ore. Mineralization evidently went on coincidently with or 

 very soon after the intrusion of the dykes during Devonian times ; 

 indeed, in New South Wales and in Queensland, where the conditions 

 are very similar, detrital gold has been worked in mechanical 

 sediments of Lower Carboniferous and Permo-Carboniferous age. 



Where the gold came from is a question not easy to answer. 

 Some of the Victorian dykes have been found on analysis to contain 

 gold, sometimes as much as ^ oz. to the ton, but it may always be 

 argued that the enrichment ot the dyke was due to the same causes 

 as that of the lodes themselves. It seems to be generally believed 

 that the mineralization took place from below ; to my mind, 

 however, the evidence seems to point to the leaching out and 

 redeposition of the minute quantities of gold present in the shales 

 by the action of heated acid waters connected with the intrusion. 

 A fact which has, however, frequently struck me is the almost 

 invariable association of gold in this class of deposit with rocks 

 containing original hornblende, and it has been noted in New South 

 Wales that in every case where auriferous reefs are associated with 

 granite the latter is hornblendic.^ Whether there is any significance 

 in this association remains to be seen, but it certainly seems worthy 

 of further investigation. 



1 Pres. Address to Geol. Assoc. 1899. 



2 " The Downward Extension of Quartz Eeefs" : Ann. Eep. Chamber of Mines of 

 Victoria, 1901, p. 36. 



3 G. F. Pittman: " Mineral Resources of N.S.W.," p. 62. 



