53 



which men of equal ability have abandoned, large profits have 

 afterwards been realized. Therefore the theory of the formation 

 of mineral veins, and the rules which lead to the discovery of the 

 richer deposits, are objects of much greater national importance 

 than is generally supposed. It is essential to the interest of 

 every mining proprietor to know the general character of the 

 local dissemination of the minerals in the district, and indispen- 

 sable to his forming a correct judgement on the mode of work- 

 ing adopted by the practical miner. 



The metalliferous deposits are subject to be decomposed and 

 recompounded periodically, according to the nature of the local 

 changes. In some situations it is possible that veins may change 

 their character in a comparatively short period, so as to be rich 

 at one time and poor at another, especially if kept full of water. 



Numerous instances may be mentioned where old workings 

 have been partially filled with a fresh crop of minerals, and also 

 where minerals have been decomposed and disappeared. After the 

 production of some crystals these are again decomposed by new 

 elements ; and thus we find crystals have disappeared after 

 having once served as nuclei for others to be deposited upon. 



A kind of efflorescence of gold, blende and pyrites have been 

 found formed on the walls in old workings in the mines of Mar- 

 mato in New Grenada. Gold washings are often abandoned, 

 and the very same sand becomes again sufficiently rich to be re- 

 washed, if situated immediately on a primary rock. Capillary 

 gold, silver and copper have been found formed in old workings 

 near Ibague, subsequently to the mines having been worked by 

 the Aborigines. Native copper has been found formed on the 

 timber in the Wicklow mines. 



Mr. W. Forster states, that at Wolfclough mine, in the county 

 of Durham, which was closed for more than twenty years, and 

 opened again, needles of white lead ore were observed projecting 

 from the sides of the veins, more than two inches in length, 

 being equal to a vein two inches wide. 



D'Aubuisson observes, that in the mines near Pontgibaud, fer- 

 ruginous and calcareous deposits are now effected in the open 

 spaces left in the mines j so that if after working out the lode 

 the galleries be left shut, and filled with the solutions of the 

 bounding rocks, during a long series of years, new workings 



