Phenomena with the Origin of Mineral Veins. 407 



Is it then unreasonable to suppose that true mineral veins 

 have originally been fissures, often produced by volcanic disturb- 

 ances, which have subsequently become filled during the ensuing 

 periods of solfataras and hot springs ? This view of the subject 

 would appear to indicate the nature of the connexion existing 

 between such veins and eruptive rocks, and also to explain why 

 i-hey should generally be of more frequent occurrence in the 

 older rocks than in formations of comparatively recent date. 



Carefully conducted analyses, executed on large quantities of 

 the waters issuing from active solfataras and thermal springs, 

 might probably afford valuable information on this subject; but 

 it must not be forgotten, as observed by M. E. de Beaumont, 

 that we cannot expect the contents of hot springs and mineral 

 veins to be identical. The more insoluble of the bodies in solu- 

 tion, both simple and compound, will be deposited in proportion 

 as the temperature and pressure decrease; the more soluble 

 only being discharged in the waters issuing from the surface*. 



The occurrence in lodes of minerals exhibiting pseudomorphic 

 forms, apparently produced by their deposition in moulds left 

 by the removal of crystals of other substances, and the presence 

 in drusy cavities of stalactites of calcite, quartz, pyrites, &c, 

 indicate that a partial decomposition and re-arrangement of some 

 of their constituents has been effected by the action of water at 

 comparatively low temperatures. There can also be little doubt 

 that fissures and cavities have sometimes been filled by infil- 

 tration from the enclosing rocks, as well as by the percolation 

 of meteoric waters from the surface. The operation of these 

 agencies is perhaps, in most instances, extremely slow, although, 

 according to E. B. Smyth, even gold, under certain conditions, 

 may be deposited in appreciable quantities within comparatively 

 short periods. This author states that, in the gold-fields of 

 Victoria, pieces of highly mineralized fossil wood, taken from the 

 deeper workings, as well as timber used for supporting galleries, 

 which had remained in the mine for some years, have exhibited, 

 under the microscope, particles of gold adhering to and intermixed 

 with crystals of iron pyrites, all through the central parts of the 

 woodf. This is confirmedby Mr. Ul rich, who says that in the gold- 

 drifts pyrites is often found incrusting or replacing roots and 

 driftwood, and that samples, assayed by Messrs. Daintree, Latta, 

 and Newbery, have yielded amounts of gold varying from a few 

 pennyweights to several ounces per ton. According to Mr. H. A. 

 Thompson, a specimen of pyrites from the centre of an old tree- 

 trunk gave by assay above 30 oz. of gold per ton J. 



* Bulletin, vol. iv. p. 1278. 



t The Gold-fields and Mineral Districts of Victoria, by R. Brough Smyth, 

 p. 74. 



% Notes on the Physical Geography, Geology, and Mineralogy of Vic- 



