224 



GRANITE. 



TJPPEE 



Galleky. 



"Wall-cases 

 6 and 7. 



22. — Fine-grained granite, 



fi 



spar, 

 shire. 



and black and silvery mica. — Strontian, Argyle- 



23. — Fine-grained granite. — Quartz, felspar, 



mica. — Strontian, Argyleshire. 



24. — Fine-grained granite. — Quartz, felspc 

 times with a tendency to form larger separate crystals), and 

 much black mica, — Strontian, Argyleshire. 



(some 



25. 



Ffelsp 



quartz, and silvery mica.— Pains Bridge, near Warleggon, 

 Cornwall. Map 30. Forms a projecting granite point, ap- 

 parently thrust in amongst the adjacent slates. Might be 

 usefully employed for architectural purposes. 



Granite. Quaternary compound, composed of 



quartz, mica 

 and a small quantity of 



26. 



felspar (two varieties), light brown 

 (two varieties), black and white, 

 schorl. — Near Giant's Punch Bowl, St. Agnes, Stilly 



Isles. 



Schorl contains about 10 per cent, of boracic acid, 39 of 



silica, 31 of alumina, a variable quantity (4 to 12 per cent.) 

 of protoxide of iron, 2 to 9 of magnesia, with a few other 

 subordinate substances, as lithia, soda, and potash. (Her- 

 mann.) 



The researches of Ebelman prove that boracic acid, 



at a high temperature, acts like water as a solvent with 

 regard to other substances. 



Hence, if boracic acid be used as a solvent, a portion 

 would be removed at a considerable heat, while other por- 

 tions of the substances held in solution by it, would crys- 

 tallize. It is on account of this easy removal of boracic 

 acid at high temperatures, that the schorlaceous portions of 

 rocks are principally situated on the outer part of the main 

 mass ; and that the skirts of masses of granite are more 

 schorlaceous than the more central portions. 



27. — Granite, composed of much silvery mica, quartz, 



felspar, a 



Cornwall. 



Map 33. — Tregencler, near Ludgvan, 



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