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



lyzed one (Antrimolite) from Antrim in Ireland, near the 

 Giant's Causeway. 



These facts favour throughout the view we urge, that the 

 amygdaloidal minerals have in general resulted from infiltra- 

 tion, and were not necessarily formed simultaneously with 

 the erupted rock. 



5. We remark farther, that no lavas have ever been shewn 

 to contain, at the time of ejection, any of the zeolitic mine- 

 rals. The zeolites of Vesuvius are known to occur only in 

 the older lavas, and afford no evidence against our position. 

 The cavities in lavas, as far as observed, are empty as they 

 come from the volcanic fires, with the exception of those 

 containing sparingly some metallic ores which are condensed 

 within them. Considering the fusibility of the zeolites and 

 their easy destruction by heat and by volcanic gases, sulphu- 

 reous and muriatic, we should, a priori^ say that they could 

 not be formed under such circumstances. 



6. Besides, as we have stated, none of the proper consti- 

 tuents of trap or basalt — or the minerals disseminated 

 through these rocks, — contain water. They are all anhy- 

 drous. The minerals formed accidentally in furnaces are 

 anhydrous. The constituents of granite, syenite and por- 

 phyry are all anhydrous. It is only those minerals which 

 are found in geodes or seams that contain water. Of equal 

 importance is the fact, that none of the essential constituents 

 of these rocks have ever been found in these geodes or cavi- 

 ties along with the zeolites, as might have been the case had 

 they been formed together, by segregation or otherwise. 

 Neither feldspar, although so abundant, nor augite, nor chry- 

 solite, have been found filling, like zeolites, or with them, the 

 cavities of amygdaloid. There is, then, a wide distinction be- 

 tween anhydrous constituents of these rocks, and the hydrous 

 zeolitic minerals. 



A few zeolites have been found in granite or gneiss, but 

 they are so disseminated that they can be shewn to be of 

 more modern origin than the rock, and to have resulted from 

 some decompositions of true granitic minerals. They differ 

 entirely in their mode of distribution from the feldspar, gar- 

 net, &c., of granite. Along with a decomposing feldspar, it 



