396 
THE GEOLOfilST. 
VI. Contents of veins varybiij vith dtrediun. — No circumstance con- 
nected with metalliferous deposits lias 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 — shotdd differ completely in their contents. Yet such is often 
the case ; and a fact so unaccountable has been the soui'ce 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 dii'ection has been estabhshed, 
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 
gi-anite was primitive ; and experience shows us that the one is as 
unfounded as the other. Analogous metalliferous deposits may have 
been produced at widely removed geological periods. When we 
speak then of older, or of more recent veins, we refer not to their nhso- 
lute but to their relaiive 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 he coni'inued). 
