THEORY OF METALLIFEROUS VEINS. 243 



7. Since veins are the fillings of fissures, they are often slipped by 

 each other or by dikes or by simple unfilled fissures. If a metalliferous 

 vein is thus slipped, according to the law of slips already given (p. 231) 

 the foot-wall of the vein has usually gone upward, and the hanging wall 

 dropped downward. The great importance of this law in practical 

 mining is sufficiently obvious. All the slips of Fig. 215, except that 

 made by the fissure c, follow this law. 



8. TJie surface-indications are to be learned by attentive observa- 

 tion in each case. We have already given these in the case of copper, 



lead, and gold. 



Tlieory of Metalliferous Veins. 



Our knowledge of the conditions under Avhich, and the chemical pro- 

 cess by which, fissures have been filled with mineral matter, is ) T et, un- 

 fortunately, very imperfect. Many vague and crude theories have been 

 proposed. Some have supposed that they have been filled in the man- 

 ner of dikes and granite veins, by igneous injection ; others, that these 

 fissures, opening below into the regions of incandescent heat, have been 

 filled by sublimation, i. e., by vaporization of certain materials and 

 their condensation in the fissures above. Some suppose that electric 

 currents, such as are known by observation to traverse certain veins, 

 have been the chief agents in the transference and accumulation of the 

 mineral matter. These three theories may be dismissed as being un- 

 tenable or else as too hypothetical. Still others have thought that great 

 fissures have filled in the same manne^ as the smaller fissures, and cav- 

 ities of every kind found in the rocks, viz., by infiltration of soluble 

 matters from the fissured rocks. There is certainly considerable anal- 

 ogy between small infiltrative veins and great fissure-veins in their 

 mode of formation; yet there is a decided difference. The fillings 

 of infiltrative veins are derived, in each part, entirely from the bound- 

 ing rock on either side. The fissure is filled by a lateral secretion 

 from its walls ; the broken rocks heal themselves " by first intention " 

 by means of a jjlasma oozing from the sides. But great fissure-veins 

 derive their contents in each part from all the strata to great depths, 

 and especially from the deeper strata. Hence the contents of these 

 veins are far more varied. 



Outline of the Most Probable Theory. — The contents of mineral 

 veins seem to have been deposited from hot alkaline solutions coming 

 up through fissures ; in other words, from liot alkaline springs. We 

 will attempt to show this first for the vein-stuffs, especially quartz, and 

 then for the metallic ores, especially the metallic sulphides. 



Vein-Stuffs. — 1. They were deposited from solutions, (a.) The rib- 

 bon-structure and the interlocked crystals (Figs. 210, 211) suggest at 

 once successive deposition from solution, especially as a similar structure 

 occurs in the fillings of cavities of all kinds, which could not have been 



