130 MIKEBAL TEINS. . 



but they are far more frequent in the first two classes. 

 They exhibit various peculiarities, many of which the present 

 state of our knowledge does not enable us to explain. Near 

 the surface they are usually poor in the metal but become 

 richer at certain depths, and poor again in lower situations. 

 They also change their metal at different distances from the 

 surface ; the same vein, in Cornwall, having been known 

 to contain zinc above, and copper below ; while there are 

 mines in the south of France which contain iron above, next 

 silver, and, lastly, copper. They are divided into two 

 kinds ; 1st, those which are contemporaneous with the rock 

 itself, and are supposed to have been formed by the sepa- 

 ration of the metallic particles from the surrounding mass 

 into one point, and are therefore termed Yeins of Segrega- 

 tion; 2nd, the metalliferous lodes, are considered to have 

 been fissures, caused during the elevation of the rocks, 

 which have been subsequently filled by metallic substances. 

 The phenomena of mineral veins constitute problems of 

 considerable difficulty, the full solution of which must be 

 left to a more advanced state of science. For the present it 

 may satisfy the student to be acquainted with those well- 

 ascertained facts which have been established; and the 

 farther consideration of the question may be reserved for 

 future inquiries. The first is their evident connexion with 

 igneous rocks and axes of disturbance, the richest mineral 

 districts are those situated in the proximity of such regions. 

 "While Becquerel and Mitscherlich, have succeeded in pro- 

 ducing crystals by electricity, Mr. Crosse, of Somersetshire, 

 has obtained calcareous spar from water which had perco- 

 lated through a limestone rock. The mode by which these 

 remarkable results were effected is highly interesting ; 

 the electric apparatus being extremely small, and kept in 

 operation for a long period ; the mode in which it is sup- 

 posed the like results are effected in nature. It is, there- 

 fore, assumed that the difference in substances found in 

 veins, and their comparative richness and poorness, have 

 been the consequence of different electric states in the 

 rocks in which they are deposited. Much light has thus 

 been thrown on the nature of these phenomena ; and 

 more information is anticipated from observations of like 

 character. 



