90 Notes on tlie Auriferous Slate Sf Granite Veins of N.8. W. 



larfi:e continuous veins wliicli act as water channels in the rocks 

 they traverse, and either through the agency of the water, or 

 some other means, portions of the contents of veins have been 

 removed, and replaced by some other mineral. Bischof mentions 

 one case where the original deposit of a vein had been entirely 

 removed — the sole trace of it left being the pseudomorphic 

 crystallisation of the mineral by which it was replaced. 



The changes occurring in the structure of rocks, known as 

 metamorphism, is a deeply interesting subject, whose study has 

 been too long neglected. But since the field work of the 

 geological surveys, now carried out, has shown that the diorite, 

 gneiss, and granite, instead of being intrusive, are only meta- 

 morphic rocks, the importance of this branch of geology has 

 been generally recognised, and we may hope that a few years' 

 work will add considerably to our knowledge of the subject. 

 We can now see what powerful agents are at work, however 

 obscure their mode of working may yet appear. In the older 

 Silurians the rocks are laminated by the cleavage forces, and 

 traversed by diWsional planes, quite irrespective of the old 

 planes of deposit, while the constituents of the rock have been 

 aggregated by new lines of force, forming bands of slate and 

 sandstone and quartz, which traverse the sedimentary planes, 

 and in many cases have entirely obliterated them. The pseudo- 

 morphic crystals afi"ord a good illusti-ation on a small scale of 

 what may be called aggregative force. On these the original 

 mineral has been replaced by a new one, atom by atom, although 

 it is a mystery to our finite senses, how the atoms of matter 

 could find their way through an apparently solid body, to and 

 from the interior of the crystal. Change in the structure of 

 rocks is constantly at work, — consolidation and decomposition, 

 alteration of crystalline character and of chemical forms, 

 aggregation into bands and nodules, and the great agent to 

 which is due the phenomena of cleavage and divisional planes, 

 whose action can be seen in every direction, but whose real 

 character can as yet only be guessed at. 



When forces producing such great changes in the rocks are 

 known to be in constant operation, it is surprising that they 

 were not first looked to for an explanation of the growth of 

 mineral veins, seeing that they readily account for most, and are 

 not inconsistent with any of the facts observed. 



Yarious theories have been propounded to explain the mode 

 in which the metals and ores have been deposited in the veins 

 — some holding that they have come from a great depth, either 

 in a molten form or else in a state of sublimation, others that 

 they have been brought in solution in the water circulating 

 through the veins. 



