ni ORR 
Quartz and Metalliferous Veins. 351 
circumstance which is perfectly grounded in the laws of che- 
mical affinity, and whose effect can be proved in the case of 
springs likewise. It is the mutual exchange, or the expul- 
sion of one substance dissolved in water by another, with 
which the water comes in contact. Just as, for example, the 
bicarbonates of lime, magnesia, oxide of iron, and oxide of 
Manganese, are precipitated by alkalies; so the same pre- 
cipitation would follow, if water containing these bicarbon- 
ates were to come in contact with minerals having alkalies 
as constituent parts; for, although the latter are combined 
with silica, still these silicates would be decomposed by the 
half-combined carbonic acid of the bicarbonate. I have 
observed such a mutual exchange in a very palpable manner. 
The mineral spring formerly mentioned as having flowed 
from under a bed of iron-ochre, had channelled a passage for 
itself in ¢rass, which was decomposed into a rich clay all 
around, and upon this channel there was an incrustation of 
carbonate of the protoxide of iron (spherosiderite). 
In this case, undoubtedly, the free and half-united carbonic 
acid of the mineral spring had extracted the alkalies from the 
silicates of the trass, whereby the bicarbonate of the prot- 
oxide of iron lost its dissolving medium, and deposited itself 
as carbonate of the protoxide of iron, as contact with the air 
was excluded. It is very probable that the carbonate of soda, 
which is so frequent a constituent of the mineral springs ris- 
ing in the crystalline formations, has this origin in many 
cases,—that, viz., water, loaded with bicarbonates of lime,, 
magnesia, protoxide of iron, and manganese, came in contact 
with formations containing silicates of soda. 
In this manner, we can easily perceive how manganese spar, 
brown or pearl spar, sparry iron, and cale-spar, which so 
frequently occur as the matrix, may have been precipitated 
from waters that contained these minerals as bicarbonates, 
so far as there were silicates of the alkalies present in the ad- 
jacent rock. This is, however, exactly the case in the me- 
talliferous veins running through the gneiss of the Erzge- 
birge, and in other crystalline formations. Such exchanges 
(Austauschungen) might be repeated more than once, if the 
