34G On the Paragenetic Relations of Minerals. 



substances constituting the minerals contained in these lodes 

 have been introduced into the fissures from the adjoining rock. 



Lodes which traverse different kinds of rocks are of dif- 

 ferent compositions in the parts adjoining those rocks. At 

 the Neue Hoffrung Gottes mine, near Freiberg, the lodes are 

 richer in ore where they traverse a very quartzy clay- slate 

 much impregnated with carbon, while in mica-slate they are 

 poorer in ore. At Konigsberg (Sweden) the largest quantity 

 of metallic silver is found in the lodes where they intersect 

 the so-called " Fallbander" beds, impregnated with argenti- 

 ferous pyrites. In the adjoining rock, close to the lodes, the 

 silver has been found imbedded, in the form of rhombic dode- 

 cahedrons. This form has probably been derived from the 

 deposition of the silver in cavities, from which garnets have 

 been removed by decomposition. 



Professor Breithaupt states, that it is a general opinion 

 among experienced miners, and supported by his own obser- 

 vation, that lodes are generally richer in ore where the adjoin- 

 ing rock is more or less decomposed. The nature and state of 

 the adjoining rock are two of the most significant among the 

 conditions of richness of lodes. There are undoubtedly many 

 other circumstances of a similar kind, which are, however, 

 known only in mining districts. 



It is difficult to perceive in what manner this secretive 

 formation of minerals can have taken place, except as the re- 

 sult of decomposition in the rocks. It is possible, and indeed 

 probable, that in some instances this secretive formation has 

 been preceded by an impregnation of the rocks with mineral 

 substances erupted or sublimed through the yet vacant fis- 

 sures. 



Some few rocks, especially granite, contain imbedded tin 

 ore. The topaz rock and porphyry of Saxony both contain 

 tin ore, as perhaps do some kinds of gneiss and mica-slate. 

 The tin ore occurs in the rock in such extremely minute par- 

 ticles, as to be imperceptible to the eye. It is frequently 

 found as a strong impregnation of the rock contiguous to lodes 

 which are quite filled, and into which it would appear to have 

 been segregated until the fissures were incapable of receiving 



