104 The Paragenetic Relations of Minerals. 



and practical point of view. As the form and origin of lode 

 fissures are treated of in all elementary works on Geognosy, 

 they may here be considered as already known. 



The crystallisation of the minerals in lodes has not always 

 commenced from the saalbands, but, in some few instances, 

 from the middle of the fissure. 



It is a remarkable circumstance that some lodes have no 

 out-crop ; and although sometimes this may be owing to a 

 subsequent formation of rock above them, there are instances 

 in which this view is inadmissible. Moreover, many lodes 

 which do crop out gradually increase in thickness downwards. 



It is a well-known fact, that the lodes in one particular 

 district have a more or less parallel direction, and those which 

 intersect at any angle either do not correspond at all in their 

 contents, or less than those which are parallel. 



Upon inquiring into the probable causes by which lode 

 fissures have been filled with minerals, it is at the outset 

 evident that they must have been very various. When a lode 

 contains only the same series of minerals constituting the 

 rock traversed by it, this rock is, with few exceptions,* 

 the source from which the substance of the lode has been 

 derived, — for instance, veins of calcite in limestone. When 

 veins of one rock traverse another — basalt in sandstone — 

 they are of eruptive origin. However, these are less frequent 

 than those which traverse only one kind of rock, in which the 

 minerals of the vein or lode either do not occur at all, or only 

 in very small proportion. 



The greater number of lodes, and those of the greatest in- 

 terest, occur in the older crystalline rocks, and especially in 

 those possessing a schistose structure, and consisting of sili- 

 cates, generally mixed with quartz. These anhydrous silicates 

 are, however, very rare in lodes themselves, while quartz is a 

 very frequent and abundant constituent of them. 



Among the anhydrous silicates occurring in lodes bear- 

 ing species of the usual ores, the felsites are most rare. A 

 few, such as pegmatolite, oligoclase, tetartine, have here and 

 there been met with. On the contrary, epidotes, especially 



The occurrence of granite veins in gneiss is such an exceptional instance. 



