and the allied Rocks. 55 



5. We remark further, that no lavas have ever been shown 

 to contain at the time of ejection any of the zeolitic minerals. 

 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 destruc- 

 tion by heat and by volcanic gases, sulphureous 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 anhydrous. The 

 minerals formed accidentally in furnaces are anhydrous. The 

 constituents of granite, syenite and porphyry, are all anhy- 

 drous. 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 cavities along with the zeolites, 

 as might have been the case had they been formed together, 

 by segregation or otherwise. Neither felspar, although so 

 abundant, nor augite, nor chrysolite, have been found filling, 

 like zeolites, or with them, the cavities of amygdaloid. There 

 is then a wide distinction between the 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 shown to be of more 

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

 decompositions of true granite minerals. They differ entirely 

 in their mode of distribution from the felspar, garnet, &c. of 

 granite. Along with the decomposing felspar it is not un- 

 usual to find stilbite in the cavities formed by the decompo- 

 sition. 



Zeolites also have been found disseminated through the 

 texture of basalt, clinkstone, &c, like the felspar, augite, &c. 

 But the proportion varies widely, and in some parts of the 

 same bed they are found to be wanting: so that we have suf- 

 ficient reason for classing these disseminated zeolites with 

 those in the cavities, as formed or introduced by infiltration. 



7. Bearing upon this subject, it should be observed, that 

 the constituents ofamygdaloidal minerals are, in general, those 

 of the containing rock. Silica, potash, soda, alumina, are 

 found in the felspars; lime, magnesia and iron, in augite or 

 hornblende ; iron and magnesia in chrysolite. These are all 



