i:m 



260 



MINEllALOG 



v/ 



[Sect. VIII. 





» 



known as eloans by the miners of Cornwall, and let the 

 annexed figure represent a section of one of them a a 



being some schistose rock broknn through or fractured 



( 



/ c 



onsolidated : granite is 



thus frequently fractured, and the fissure filled by an 

 elvan.) We find that while the central portion d may be 

 a granite, the parts c c are porphyritic, and h h some com- 

 pact rock. Upon investigation, we see that all parts are 

 chemically th- same, and that these various characters 



are due to differences in coolin 



The central portion 



dift 



erences of 



ooling can be imitated 



retained its beat longest, while the portions adjoining the 

 bounding and fractured rocks were more speedily cooled. 

 In such porphyries various minerals are found, those of 

 the felspar family being very common. Such results from 



artificially with 

 substances under our control. In this way crystals of 

 silicate of lime may be beautifully obtained, isolated in 

 transparent glass. 



Whole mountain masses are occasionally composed of 

 porphyritic rocks, including the porphyritic granites among 

 them ; and it is desirable to obtain specimens of these, 



selecting portions where the crystals may be well formed, 



and observing, should more than one kind of isolated 

 mineral be present, how far when one kind becomes com- 

 mon another may disappear, and if different kmds continue 

 mixed through the general m..s, or only in patches. It 



I 



I 

 S 



nnl 



of*" 



beefl 





1 n 



r-T"-"?]! 



i-- 



i 



It 



onoe 

 look 



• 1 



or;' 



SubSi 



by p. 



beai 



T: 



have 

 oftl 



igne 



intei 



adju 



this 



of 

 is 



ISO 



tio 

 fro 



