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'410 





I 



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Sect. VIIL] 



JVIINERALOGY 



261 



is equally desirable to obtain good characteristic specimens 



of 



been uninjured by exposure to the weather, and have lost 

 little of the soluble substances which may have once 

 been contained in the rock. 



The chemical study of th^ 



whole of such igneous rocks is every day bccomin 





more 



interesting. 



It is not only among the igneous rocks which have 



hould 



once been in a molten state that the observer 



look for minerals, but also, in volcanic regions, for those 



evidently sublimed upon the faces of craters, or in cracks 



chink 



Many 



substances thus obtained are difficult of preservation ; but 

 by putting them away in bottles, well stopped, much may 



be accomplished. 



The minerals often seen isolated in those rocks which 

 have been termed metamorphic, or altered, in consequence 

 of the upburst or protrusion of some rock in a state of 

 igneous fusion near them, constitute a class of much 

 interest. Here again we see conditions well fitted to the 

 adjustment of the integrant molecules of minerals ; but 

 this case so far differs from that of the porphyries, that 

 whereas in the latter the whole mass has evidently been in 

 a fluid or viscous state, in the former the stratified cha- 

 racter of the rocks of that class is preserved. The manner 

 of observing this order of rocks belongs to geology, 

 is only necessary here to call attention to the kind oi 

 isolated minerals found. Among them staurolites, anda- 



■ 



lusites, and garnets are frequent under certain condi- 

 tions, which it may be advisable to guard the observer 

 from supposing merely those of temperature. The free- 



n - 





s^ 



kk 



