226 



ON GRANITE, 



t*a 



On the name 

 granite. 



the water has funk gradually and calmly. They ufually oc- 

 cupy the higher parts of the globe, but when covered with 

 newer ftrata may form the loweft. The rocks which Werner 

 considers as belonging to this great clafs are, granite, gneifs, 

 mica flate, primitive flate, porphyry, and fienite. 



As granite is in many refpecls one of the moil important of 

 thefe formations, I fhall here detail feveral interefting particu- 

 lars refpecling it. 



Fliny, and the older writers, defcribe this rock under dif- 

 ferent names j the term granite appears to be of modern date, 

 as Montfaucon is the firft writer who ufes it. This will not 

 furprize us, when we confider, that it was not until the re- 

 vival of letters when the remains of antiquity began to be flu- 

 died, that the different rocks received particular denomina- 

 tions. To Werner we are indebted for the molt exacl: defcrip- 

 tion ; before his time it was confounded with fienite and 

 grunftone, two rocks that differ both in their oryctognoftical 

 and geognoftical characters. 



Granite is an aggregate, granular, primitive rock y which is com- 

 pofed of felfpar, quartz, and mica. 



Aggregate com- Felfpar is generally the prevailing, and mica the leaft con- 

 sent parts of fiderable ingredient. The felfpar has a confidcrable range of 



granite; felfpar, to . r j j r 



quartz, and colour; the principal colours are white, grey, red, and tome- 

 mica# times, though rarely, green : it is found in all the intermediate 



ftates, from very great to very fine grain. The quartz and 

 mica are generally grey, and the firft has fometimes a black 

 colour. Several foffils occur in granite befides thofe we have 

 juft mentioned ; thefe are fhorl and garnet : fuch varieties 

 have received particular names, but the geognoft views them 

 as accidental, and does not take particular notice of them. 

 The topaz, which is diftinguiftied from all other precious 

 ftones (excepting the emerald and garnet) by its occurrence 

 in primitive mountains, is found accompanied with apatite in 

 the tin beds which lie in granite at Ehrenfredersdorf. 



Itsjiruclure is -not fubjeel to much variety. When cryftali 

 of felfpar are immerfed in a bafts of fine grained granite, it 

 conftitutes what is termed porphyritic granite. Of this there 

 are fine examples near to Carllbad in Bohemia, and in many 

 places in the north of Scotland. If fometimes occurs in glo- 

 bular 



Structure. Por. 

 pbyritic. 



Globular, 



