396 



THE GEOLOGIST. 



VI. Contents of veins vanjing with direction, — No circumstance con- 

 nected with metalliferous deposits has been productive of more diffi- 

 culty than this. Wliether filled from above, or below, or by segregation, 

 it seems at first sight unaccountable how two sets of veins in the 

 same district — one running north and south, and the other east and 

 west — should differ completely in their contents. Yet such is often 

 the case ; and a fact so unaccountable has been the source of more 

 mysticism than any other connected with the theory of ore-veins. It 

 has been freely attributed to some occult action of electricity, or 

 " polar forces," whatever that phrase may mean, and similar hypo- 

 thetical causes. It must be understood that no general relation 

 between the contents of veins and their direction has been established, 

 although it seems to be asserted by some persons that such a relation 

 does exist. The only known relation is a local one. For instance, 

 in Cornwall and Devon lead-ores are contained in north and south 

 veins, and not in east and west veins ; whereas, in the lead- district 

 of central Wales the ores of that metal are found abundantly in east 

 and west veins. 



VII. Theory of Vein '■''Formations.'' — According to this theory veins 

 of certain classes were considered essentially characteristic of certain 

 geological ages — were absolute " formations" in its geological sense 

 of synchronous. It supposed special periods in the earth's history to 

 have been marked by special metallic emanations which entirely 

 passed away with those periods, of which they were characteristic. 

 This doctrine may be compared to the very similar one that all 

 granite was primitive ; and experience shows us that the one is as 

 unfounded as the other. Analogous metalliferous deposits m.ay have 

 been produced at widely removed geological periods. When we 

 speak then of older, or of more recent veins, we refer not to their abso- 

 lute but to their reloMve age in the same locality ; or, when speaking 

 of widely removed localities, we only refer to age with reference to 

 other veins in each respective locality, 



(To be continued). 



