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Sect. VIIL] 



MINERALOGY. 



Tip, various mineral substances are obtained well crystal- 

 lized. It may be here observed, as resards the metallic 



titanium frequently discovered in iron furnaces when 



blown out, that we have found the oxide of 

 crystallized in the cavities of clay ironstones. Taken as 

 a whole, the observer will do well to look into any cavities 

 or cracks he may discover in rocks, even in the hollows 

 among organic remains, for various mineral substances. 

 Many a crystallized body will thus be frequently found, 

 and the replacement of one substance by another be well 



seen. 



Not only in cracks or hollows, but in the body of the 



rocks themselves, minerals may be observed well crystal- 

 lized, their component molecules having had free power 

 to adjust them.selves according to the affinities and forms 

 proper to them. This is well seen in the class of igneous 

 rocks known as porphyries— that is where a general paste 

 or base, confusedly crystalline, compact or earthy, may 

 happen to contain isolated and well-formed minerals of dif- 

 ferent kinds. From experiments in the laboratory, and the 

 results of metallurgical and chemical operations carried 

 on upon the large scale, we know that this isolation of 

 crystals may readily be obtained, it being merely needful 



that the conditions for their production should be such that 

 their component particles could freely adjust themselves 

 first in the cooling mass. In the igneous dykes, as they 

 are termed — that is, where igneous matter in fusion has 

 been forced up, filling cracks formed in the rocks which 

 they traverse — we sometimes see good illustrations of the 

 mode in which isolated mineral crystals may be produced. 



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take as an examnle some of the granitic 



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