MINERAL VEINS. 



Fig. 43. 



d/ 



Fig. 44. 



great numbers prevail in the same system of strata, and generally 

 take a direction nearly parallel. Fig. 43 shows a transverse 



section of such a sys- 

 tem. The similarity 

 of the mineral matter 

 which fills them de- 

 monstrates their com- 

 mon origin. It often 

 happens that one sys- 

 tem of such veins is 

 intersected by another, 

 presenting mineral 

 contents totally differ- 

 ent from the former. 

 These are called cross 

 veins. It is rare that 

 a vein is completely 



filled with metalliferous matter. Most commonly, these sub- 

 stances form threads, , 6, c, d, e, /, g, fig, 44, more or less 

 irregular, included in the middle of the stony crystalline matter 

 which fills the vein. The thickness of 

 the metalliferous thread varies at dif- 

 ferent parts of the vein ; at some points 

 it is considerable, and at others be- 

 comes very small, often vanishing 

 altogether. 



106. Numerous cavities are often 

 formed in the midst of stratified rocks, 

 probably by the dissolving action of the 

 subterraneous waters. Such cavities, 

 which are met with in all the sedi- 

 mentary strata, are generally filled after 

 their formation with new substances 

 totally different from the surrounding 

 rock. It is thus that masses of rock- 

 salt are found in the Muschel-kalk 

 and in the Marnes Irisees, fig. 45. 



Similar masses of carbonate of zinc, 

 as formed in the upper part of a stra- 

 tum of transition limestone, are shown 

 at c c, in fig. 30. 



107. Since the memorable revolution 

 effected by Bacon in the conduct of phy- 

 sical inquiries, a maxim has been uni- 

 versally admitted and adopted, in virtue of which, in the formation 



85 



