STOCKWORKS. 405 



floor of this kind has been very extensively worked in a gneiss 

 rock. 



The stockwork of the German miners is to be considered as a 

 mass of rock impregnated with metallic matters, in numerous small 

 veins, which come together irregularly, so as to make particular parts 

 extremely rich. The working of such mineral repositories is directed 

 by quite other principles than those which serve for straight veins 

 of definite magnitude. The stockwork is generally opened like 

 a vast quarry, and the excavations are prosecuted irregularly in 

 the most favourable directions. Perhaps the copper mine of Parys 

 mountain in Anglesea, the iron mine of Dannemora in Sweden, 

 the tin ore mine of Geyer in Saxony, are examples of immense 

 stockworks. Werner considered the stockwork as peculiar to tin 

 ores. 



Eelations of Veins to each other. The influence which veins 

 exert on each other may be in some measure ascertained by an 

 examination of the phenomena at the points where they come into con- 

 tact with or cross each other. At these points it is very often found 

 that the quantity of ore is suddenly increased to a large amount, and 

 for some distance, in either one or both of the veins. Many veins 

 are productive only near such points, or yield there peculiar ores and 

 minerals. This does not depend upon the enlargement of the vein 

 merely, but may indicate the influence of thermal or electric condi- 

 tions in the disposition of the materials of mineral veins. We have 

 heard miners say that in certain cases neighbouring veins are subject 

 to a kind of reciprocity, so that they are not both productive in 

 the same ground, but where one is rich the other is poor. 



Age of Veins. When two veins cross, it almost invariably 

 happens that one of these cuts, or is continued, right through the 

 other, as a wall is sometimes continuous through another wall of brick 

 from top to bottom. Thus a vein of copper ore may cross and cut 

 through a vein of tin ore ; a vein of lead ore may cut through a vein 

 of copper ore, and all these be cut through by some other sparry vein 

 or porphyry dyke. It is supposed, by almost every writer on the 

 subject, that the relative antiquity of the veins which thus intersect 

 one another may be immediately determined ; and that in every case 

 the vein which is cut through is the older of the two. Werner took 

 this as the basis of his classification of veins, and most practical as 

 well as theoretical miners agree in his views ; but they are neverthe- 

 less controverted. 



We cannot make a step in this argument, except upon the admis- 

 sion that the veins are posterior in date to the rock which encloses 

 them ; in other words, that the space in which the mineral masses of 

 a vein lie, once existed as a fissure in the rocks, and was subsequently 

 filled up by the accumulation of the sparry and metallic matters. 



The Neighbourhood of a Vein. It is a general fact that the walls 

 of a vein partake in some degree of its characters, and that effects, 

 apparently depending on the vein, propagate themselves into the 

 neighbouring rocks. Thus the walls become more indurated, more 



