Geology and Magnetism. 529 



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 pro- 

 duction of some crystals these are again decomposed by new ele- 

 ments ; 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 Marmato 

 in New Grenada. Gold washings are often abandoned, and the 

 very same sand becomes again sufficiently rich to be rewashed, if 

 situated immediately on a primary rock. Capillary gold, silver and 

 copper have been found formed in old workings near Ibague, subse- 

 quently 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 effective in the open 

 spaces left in the mines ; 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 could be carried 

 on upon the new deposits. The rubbish left in old workings be- 

 comes often cemented by mineral salts, which sometimes crystallize 

 in the crevices, so as to render it worth working over again in the 

 course of a very few years. In the mines of Hanover a leather 

 thong suspended from the roof of a mine was found coated with 

 silver ore, and also native silver and vitreous ore coating the wooden 

 supports of a mine which had been under water for several years. 



These chemical actions, governed by the subterranean polar cur- 

 rents, continue to fill every fissure or vacuity with crystals, the 



