394 
THE GEOLOaiST. 
question is the first difficulty we have to grapple with, and it has 
been fruitful of hypotheses. The Wernerian theory that the contents 
of veins (the most common form of metalliferous deposit) were de- 
lived from above has been universally abandoned. There remain 
two othei's : one, that these deposits originate by segregation fi-om 
the neighbouring rock ; and the other, that they are derived, by some 
means or other, from beneath. The segi-egation theory is one well 
worthy of consideration, and undoubtedly applies to many metal- 
liferous deposits ; but it stUl leaves unsolved the main problem of the 
original som-ce of the metals ; for it completely fails to account for 
the abundant distribution of the metallic ores in some districts, while 
in others, in the same class of rock, they are entirely absent. To ac- 
count for all metalliferous deposits by segregation, irrespective of any 
other cause, we should have to have recourse to the alchemic doctrine 
of the possible transmutation of earths into metals. The theory of 
the metalHc ores from beneath, has been suggested in every foi-m ; 
and is not without many difficulties to which I shall refer further on. 
III. Various forms of mctaJliferous deposits. — Next to the question 
of the metallic ores, we are met with difficulties depending on the 
forms in which they are usually found. They generally occur in 
abundance in veins of a definite size and direction — the direction being 
constantly associated, in certain districts, with chai'acteristic ores. 
But they are also found in forms that are not veins, such as in 
" stocks," or in uTegular and indefinite masses. Veins are not 
peculiar to ore-formations, but are found in almost every rock, and 
include every kind of mineral species. Still, as from their nature, 
veins are liidden far from our sight ; we only become intimately ac- 
quainted with them when they consist of such minerals that the 
necessities of man lead to their exploration. Hence our knowledge 
of veins is principally confined to those containing ores, or other veins 
associated with them ; and we are often consequently led, very 
erroneously, to imagine that no veins exist but ore-veins, or those 
connected with them, and to limit the occuiTence of minerals in the 
peculiar /o'/^rt we denominate veins exclusively to minerals of the use- 
ful metals : an idea productive of considerable misapprehension and 
confusion. 
IV. Most mineral-veins ivere fasures, suhsequeutli/ filled. — This is 
