OBSERVATIONS ON E.is.lLT, &C. 1()7 



ordinary rock called the globulai Granite of Corfica, is an Globular granite 



analogous in fiance. It is compofed of cryilals of hornblende, a 



feldfpar, quartz, and mica, in confufed aggregation ; and in this 



bails are immerfed fpheroids, about an inch and a half or two 



inches in diameter, compofed of concentric alternate coats of 



quartz and hornblende. The centre is principally occupied by 



hornblende; this is furrounded by a zone of quartz. The fa 



fpheroids are radiated to the centre. There can be little doubt 



that this rock is merely the refull of interrupted cryfiallization; 



and that, if the procefs of arrangement had continued, this 



ftruclure would have difappeared, and the whole rock would 



have refembled the prefent bahs. Hitherto, this very lingular 



rock has only been found in detached fragments.* 



The ad mi Mi on that folution is not a requifite of cryfialliza- Solution not be- 

 tion, appears to me an important concelhon in favour or the t0 ° ryftallization 

 aqueous fyfiem, which has laboured under very great embar- butonly mecha- 

 raifment, from the difficulty of diflblving quartz. If a very ™" 1 fB ^S| 

 perfect mechanical iulpenfion be all that is requifite, we may cifficulties are 



removed. 



* I (hall venture to quote another inftance, on the authority of 

 Profcflor Playfair. " The fait rock in Chefhire, which lies in thick 

 " beds, interpofed between fti ata of an argillaceous or marly ftone, 

 << and isitfdf mixed with a confiderable portion of the fame earth, 

 (i exhibits a very great peculiarity in its ftru£ture. Though it 

 " forms a mafs extremely compact, the fait is found to be arranged 

 *« in round maffes, of rive or fix feet in diameter, not truly fpherical, 

 " but each comprefTed by thole that furround it, fo as to have the 

 " ffiape of an irregular polyhedron ; thefe are formed of concentric 

 " coats, diftinguifhable from each other by their colour, that is, 

 " probably, by the greater or lefs quantity of earth which they con- 

 " tain ; fo that the roof of the mine, as it exhibits a horizontal fec- 

 »> tion of them, is divided into polygonal figures, each with a mul- 

 <c titude of polygons without it, having altogether no inconfider- 

 " able icfeinblance to a Mofaic pavement. In the triangular fpaces 

 " without the polygons, the {alt is in coats, parallel to the fides of 

 " the polygons." Illuftration of the Huttoniarj Theory, page 37. 



I am informed, that the filiceous deposition at Geyier, is at fir ft 

 a porous friable mafs, and that the addition of more molecules ren- 

 ders it fibrous ; alfo that, on a farther addition, the fibrous ftruc- 

 ture diiappears, and the whole aflumes the compact even texture of 

 chalcedony or flint. If I am not rnifinformed, a feries of Speci- 

 mens illuftrating this traniltioB, exifted in lira cabinet of the late 

 Dr. Hutton, of Edinburgh 



ceafe 



