62 J. D. Dana on the Minerals of Trap and the allied Rocks. 



this association of the lime zeolites with a large supply oflime 

 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 — their 

 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 happen in 

 the case of 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? 



There may be instances of this mode of formation ; but that 

 it should be the usual mode is irreconcilable with the many 

 facts stated. The form and condition of quartz or chalcedony 

 in geodes, as well as the vast amount of this mineral in some 

 cases, — the relative positions of the zeolites, and their occur- 

 rence as incrustations on rocks, or as fillings of cavities or 

 seams, and never in disseminated crystals through the texture 

 of the rock, — the green coating of the nodules, which is some- 

 times a carbonate of copper when there is native copper in 

 the rock to undergo alteration, — the correspondence between 

 the elements of the minerals and the composition of the in- 

 cluding rock, and at the same time their contrast in being 

 hydrous while the constituents of the latter are anhydrous, — 

 and the known formation of zeolites in caverns, — these various 

 facts appear to establish infiltration as the principal means by 

 which amygdaloidal minerals have been produced. 



