Quartz and Metalliferous Veins. 129 



tlie trap into clay, by decomposition of its alkaline silicates, 

 and, instead of depositing silica, deposited cai'bonate of the 

 protoxide of iron. Whether, however, the deposit of silica 

 in veins resulted from abstraction of carbonic acid, as in the 

 deposit of sphserosiderite, is doubtful, as carbonic acid is not 

 the solvent of silica in water ; for when the carbonic acid is 

 expelled from mineral waters, the silica is not precipitated. 



Everything concurs to justify the assumption, that the 

 adjacent rock of veins has had a material influence on the 

 deposition of the substances fi'om the fluids which had for- 

 merly flowed in them, and that it has thereby undergone 

 many changes ; here silicification, there weathering — ^liere, 

 again, discoloration, there colouring, &c, 8ic. If it can be 

 proved, that not only the non-metallic veins, but also the 

 ores, have been introduced into the vein-clefts in the wet 

 way, as I shall endeavour to shew elsewhere, then we may 

 presume that the nature of the adjacent rock has had an 

 influence not only on the former, but also on the latter. 



In this manner, too, may be explained the so frequent 

 occun'ence, that metalliferous veins, running through differ- 

 ent geological formations, change their nature ; in one be- 

 coming richer, in another impoverished. The Erzgebirge 

 presents many examples in this respect. Thus, the veins of 

 the Halsbriicke formation occur in gneiss, less frequently in 

 mica-slate, &c. When they reach the clay-slate, they are 

 scattered, or clean cut ofl". It is exactly the same in the veins of 

 theBraunsdorffonnation; they occur in mica-slate and gneiss, 

 and lose themselves in clay-slate. The veins of the Zscho- 

 pau formation present themselves chiefly in mica-slate and 

 clay-slate ;* and here they become impoverished in the latter. 

 On the other hand, there are examples of the vein stuff" of 

 some veins appearing independent of the adjacent rock. 

 Thus the veins of the Schneeberg formation occur principally 

 in clay-slate, generally, however, in mica-slate. They run 

 out of the clay-slate unaltered in the ore they bear into the 

 granite, t 



It is very easy to conceive, that now this, now that sub- 



* Freiesleben, pp. 20, 34, 3G. t lb., p. 39. 



VOL. XXXIX. NO. LXXVII— JULY 1845. I 



