Ott GRANITE. 227 



fcnlar diftincl concretions *, and thefe are again composed of 

 Concentric lamellar, diftincl concretions. This ftru&ure of 

 granite is only to be difcovered, after the fofter granite has 

 Weathered out ; then thefe concretions which are vaftly harder, 

 and are only feparated from each other by the loofer granite, 

 make their appearance. Upon the road between Drefden and 

 Bautzen I obferved many fine examples of this ftru&ure of 

 granite : Mr. Barraud, in his interefiing defcription of the 

 Cape of Good Hope, mentions feveral globular diftincl con- 

 cretions of immenfe fize. In Scotland the ifland of Arran af- 

 fords inftances of this kind f. 



It is frequently obferved diftinclly ftratified : in other in- Diftinclly h™. 

 fiances owing to the thicknefs of the ftrata, this ftru&ure is tlfied% 

 difficultly obfervable, and has given rife to the opinion that 

 fuch granite is not ftratified. The Riefengebirge, which fe- 

 parate Silelia from Bohemia, are for 150 miles compofed of 

 granite, difpofed in horizontal ftrata. Laft fummer I exa- 

 mined thefe mountains along with a confummate mineralogifr, 

 Dr. Mitchell, and we convinced ourfelves of the truth of this 

 obfervation. I have obferved fimilar Gratification in Saxony 

 and Lufatia. 



It is an interefting fa£t in the natural hiftory of granite, that It feldom con- 

 it feldom contains extraneous beds, and Werner remarks, that f ai " s extraneou s 

 the frequency of fuch beds increafes with the newnefs of the 

 formation : thus gneifs contains fewer beds than mica flate, 

 and mica flate fewer than primitive flate. 



Limeftone, which accompanies all the newer primitive No limeftone. 

 formations, is intirely wanting in granite. 



Metals which occur in Granite. 



This rock is not fo rich in metals and their ores, as the pri- Metals which 

 mitive rocks of newer formation. It contains, however, a occur in granite; 

 confiderable variety, and fome of thefe have been as yet only 

 difcovered in granite. Iron, which is remarkable on accounti ron% 

 of its occurrence in every period of the earth's formation, is 

 found interfperfed in the oldeft granite. Red iron ore occurs 



* Thefe globular diftinft concretions ufed formerly to be con- 

 fidered as bowlded ftones, and afforded an invaluable opportunity 

 for the framing of extravagant hypothefis. 



t Mineralogy of the Scottifh Ifles, vol. i. p. 42, 



Q2 in 



