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MINERALOGY 



257 



tlie cavities that have decomposed or Injured the mineral 



• 1 ft 



substances in them. Th 



only to the depth of a few inches, but to many feet : 

 the vesicles of some igneous rocks, for example, having 

 been completely emptied, near the surface, of the mineral 

 matter which once filled the cavities, and which still fills 

 those beneath, so that externally the rocks present much 



as 



ii 



and viscous state. 



Minerals of the zeolite family are very common in the 

 vesicular canities of some igneous rocks ; and at one time, 

 before their mode of occurrence was properly understood, 

 the quantity of water found in many of them was thought 



Igneous 



■6 



a 



They form an interesting class of minerals, and, oppor- 



tunities offering, should always be collected. 



They come 



under the head of hydrated aluminous silicates, with 

 potash, soda, lime, and their isomorphons substances. The 

 great proportion of them contain from 8 to 18 per cent. 

 of water in combination. In the same kind of vesicles, 

 siliceous deposits in the form of agates are not uncommon. 

 In these and in cavities of various rocks, even those of 

 aqueous origin, such, for example, as the dolomitic rocks 

 of the new red sandstone series, in Somerset and Glou 

 cestershire, the agate linings of the cavities 



have con- 



tinued only for certain distances, after which the elements 

 of other minerals have entered the hollows, and various 

 crystallized substances have been the result. Cavities, 

 therefore, in all rocks may be searched. With respect 

 to the successive siliceous coatinecs forming agates, 



some kinds of coatings show an adjustment to the walls 







