Mr J. D. Dana on the Origin of Trap Minerals. 271 



melsfurst mine near Freyberg. It consisted of silica, oxides 

 of iron, and manganese and water. Further examination will 

 probably bring more of these modern products to light.* 



The formation of particular minerals in certain regions de- 

 pends, 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, Laumo- 

 nite, 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 decomposition 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 — ^tlieir 

 proceeding from vapours rising with the erupted rock ; for it 

 seems to be of but limited influence. Besides the arguments 

 already brought forward, we state that the vapours which rise 

 at the moment of eruption are insufficient. They inflate the 

 rock, or blow up the cavities ; but the little vapour required 

 to open the cavities most assuredly could not afford, by con- 

 densation, the mineral matter necessary to fill them, — to pro- 

 duce stalactites, stalagmite, and successive layers of minerals. 

 The vapours, then, if the source, must have continued to rise 

 for some time afterward. But is it possible that vapours 

 should rise up through the solid rock % Such does not hap- 

 pen about recent volcanoes ; for fissures are first opened, and 

 then the vapours escape. And could it happen with the water 

 above pressing down into the rock with the force of an ocean 

 even a mile deep ? 



* Carbonate of iron seems never to form from water at the surface, its solu- 

 tions depositing a hydrated peroxide of ii*on instead of the carbonate: it may 

 therefore require a submerged condition of the rock, although not necessarily a 

 raised temperature. 



