On the Minerals of Trap and the allied Rocks. 63 
described a modern stellated zeolite forming incrustations on the 
pump-wells of the Himmelsfurth mine near Freyberg. It consisted 
of silica, oxyds of iron and manganese and water. Further examin- 
ation will probably bring more of these modern products to light.* 
The formation of particular minerals in certain regions, depends 
of ‘course upon the supply of the necessary ingredients. Where 
the supply of lime has been large, we should expect to find some of 
the minerals, Prehnite, Heulandite, Laumonite, stilbite, scolecite, 
dysclasite, chabazite, for carbonate of lime decomposes the silicates 
of potash or soda. Instances of this association of the lime zeolites 
with a large supply of lime in the vicinity are common. When 
there is little or no lime, or only the results proceeding from the 
decomposing rock, the other zeolites are formed—the hydrous 
silicates of alumina and potash or soda, occasionally with some 
lime. But if a salt of baryta or strontia is present, the decom- 
position of the silicates of the alkalies takes place as by the lime, 
and the mineral harmotome or Brewsterite is produced. 
In the above explanations we have scarcely appealed to one 
source of amygdaloidal minerals admitted in the outset—their pro- 
ceeding from vapors rising with the erupted rock ; for it seems to 
be of but limited influence. Besides the arguments already 
brought forward, we state that the vapors which rise at the mo- 
ment of eruption are insufficient. They inflate the rock, or blow 
up the cavities; but the little vapor required to open the cavities 
most assuredly could not afford by condensation the mineral matter 
necessary to fill them,—to produce stalactites, stalagmite and 
successive layers of minerals. 'The vapors then, if the source, 
must have continued to rise for some time afterward. But is it 
possible that vapors should rise up through the solid rock ? Such 
does not happen about recent volcanoes ; for fissures are first open- 
ed and then the vapors escape. And could it happen with the 
water above pressing down into the rock with the force of an 
ocean even a mile deep? ; 
There may be instances of this mode of formation ; but that it 
should be the usual mode is irreconcilable with the many facts sta- 
ted. The form and condition of quartz or chalcedony in geodes, 
as well as the vast amountof this mineral in some cases,—the rela- 
* Carbonate of iron seems never to form from water at the surface, its solutions 
depositing a hydrated peroxyd of iron instead of the carbonate: it may therefore 
require a submerged condition of the rock, although not necessarily a raised tem- 
perature. 
