292 



FORMATION OF VEINS. 



In what manner metallic veins were filled with ore has greatly di- 

 vided the opinions of geologists. Dr. Hutton supposes that both 

 dykes and veins were filled with their contents in a state of fusion by 

 injection from below ; the expansive force of the melted matter hav- 

 ing cracked the surface, and opened a passage for its reception. 

 (See Chap. IX.) From circumstances previously stated, it appears 

 probable that many dykes were so formed. Other dykes appear to 

 have been open fissures filled by materials washed from the surface, 

 and contain rounded stones, and sometimes undecayed vegetable 

 matter. From a dyke of clay in a coal mine in Yorkshire, two hun- 

 dred and fifteen feet deep, I have drawn out long vegetable fibres, 

 apparently roots, the woody part of which was unchanged, and burn- 

 ed like the roots of common weeds. Werner supposes that all veins 

 and dykes were first produced by the shrinking of the materials, of 

 which mountains are composed ; and that metallic veins have been 

 filled from above by the ores in a state of solution.'^ This theory 

 has been advanced with much confidence, and warmly supported by • 

 many geologists : but I have no hesitation in asserting, that it is de- 

 monstratively repugnant to fact : indeed, the implicit credit which 

 has been given to Werner's dogmas on this subject, is one instance 

 among many, in which men of distinguished talents have resigned 

 their judgment to authority, and supported the most absurd proposi- 

 tions, when conformable to a favourite hypothesis. If veins were • 

 filled by metallic solutions from above, these solutions must have cov- 

 ered the highest mountains over the whole earth ; and, instead of ^ 

 finding metallic ores in the present confined repositories, they would j| 

 fill all the cavities and valleys in every part of the world. As this ' 

 theory supposes, likewise, that veins were formed at different times, 

 a number of these metallic solutions would succeed each other, we 

 should find regular strata of ore in all primary and transition rocks ; 

 and the quantity formed by these deep seas of metallic matter, would 

 be inconceivably great. 



This theory is, decidedly, invalidated by the following facts. 

 When a metallic vein passes through different kinds of rock, it is 

 generally observed, that the quality of the ore varies with that of 

 the rock through which it passes ; and even some beds of the same 

 rock are more productive than others, and are called by miners 

 hearing measures. This is the case in Durham, Derbyshire, Corn- 

 wall, and probably in every mining district, in England and Wales. 



Not only does the variation in the nature of the rock, occasion a 

 change in the quantity or quality of the ore, but the mineral sub- 

 stance or matrix which accompanies ores, generally varies in differ- 

 ent kinds of rock. In granite and slate rocks, the matrix is more 



* The round pebbles which are sometimes found in veins have been cited to 

 prove that veins were filled from above : they were probably introduced by subter- 

 ranean currents. 



