4o6 "CONTEMPORANEOUS VEINS" NOT METALLIFEROUS. 



crystalline, and for considerable distances are filled with the matters 

 of the vein ; and even the very substance of the rocks is often impreg- 

 nated with mineral combinations. In a country where the veins are 

 numerous, large masses of the rocks may in this way be ridered, as it 

 is termed in the North of England ; and if such a gradation of char- 

 acters could be relied on as a proof of contemporaneity of origin, this 

 may in a few cases lead to the conclusion that the veins and rocks are 

 coeval. 



But the true conclusion on this point is that these effects are 

 locally related to veins; the ridering of the neighbouring rocks is 

 coeval with the production of the vein ; but since these rocks are 

 clearly defined from the veins, and fragments of them are enclosed 

 in the veins, and the mineralising influence which they have suffered 

 obviously depends on the influence of the veins, we cannot hesitate 

 to admit that these latter are of separate and subsequent origin. 



Veins in Different Eocks. It is found that when veins divide 

 different sorts of rocks, their contents vary in an inconstant manner, 

 according to the nature of the rocks. The most usual notion on this 

 subject is, that the veins may be viewed as secretions from the rocks ; 

 and by some this is supposed to have happened after the production 

 of fissures ; by others, by a mere internal separation of the parts of the 

 mingled metallic and earthy mass. 



This notion of the slow separation of the ingredients of rocks is in 

 accordance with the principles and facts of chemistry, and must be 

 often appealed to, if we would explain by true causes the phenomena 

 of mineral veins. 



Contemporaneous Veins. There are combinations of minerals in 

 masses of various figure, which, upon very good grounds, are admitted 

 to be contemporaneous with the rocks in which they lie ; and if we 

 choose to call by the name of veins all such distinct combinations of 

 minerals, these certainly are contemporaneous veins. When in granite, 

 greenstone, &c., we find particular portions of those rocks either 

 linear, tabular, globular, or in any other figure, which have a different 

 proportion of ingredients from the other parts, and in consequence 

 become conspicuous and distinct, except at the edges, which graduate 

 without any sign of fissure into the ordinary mass of the rock ; 

 these may certainly be pronounced contemporaneous veins, and they 

 have been produced by a process of secretion or segregation during the 

 crystallisation of the rock. 



In some instances veins of calcareous spar or other minerals lie 

 wholly included in limestone masses, and these are properly called 

 veins of segregation ; but they are not contemporaneous veins, for they 

 have clearly been fissures filled at some period since the consolidation 

 of the rock; and the proof is, that shells, corals, &c., are split, and 

 sometimes displaced by these sparry veins, which undoubtedly occupy 

 cracks left by the shrinking of the rock in the process of consolidation. 



Allowing every just latitude to the doctrine of contemporaneous 

 veins, we must admit that most veins are newer than the rocks which 

 enclose them and have yielded their minerals. 



