202 PHYSIOGRAPHY. 



Feldspar. 



the particular state, in which the varieties of common Feldspar occur, 

 those which are more or less decomposed, were designated by the denom- 

 ination of earthy common Feldspar, and considered as a particular sub- 

 species. If the decomposition has arrived at its limits, so that the whole 

 is converted into a more or less firmly coherent powder, Porcelain- 

 Earth is formed. It is possible that porcelain-earth arises from the de- 

 composition of several species of the Feldspar family. . 



Before the blow-pipe, upon charcoal, Feldspar becomes glassy, semi- 

 transparent and white, but melts with difficulty, and only upon its edges, 

 into a semi-transparent, vesicular glass. It is dissolved by borax, but 

 slowly and without effervescence, into a clear globule. It is not acted 

 upon by acids. 



Analysis. 

 By VAUQTJELIJY. By KLAPROTH. 



Adularia. Common Feldspar fr. Carlsbad. 

 Silica . 64 JO . . 64-50 



Alumina . 20-00 ! . 19-75 



Potash . 14-00 . . 11-50 



Lime . 2-00 . , a trace. 



Oxide of iron . . 0-00 . . 1 75 

 Water . 0-00 . . 0-75 



Feldspar frequently enters into the composition of rocks, and consti- 

 tutes, with Quartz and Mica, the different kinds of granite and gneiss ; 

 with Hornblende, it forms syenite and greenstone ; and with Augite, the 

 Augite-rock. To several of these rocks, large crystals of Feldspar im- 

 part a porphyritic appearance ; and it is a characteristic mark of the dif- 

 ferent kinds of porphyry more properly so called, to have isolated crys- 

 tals of this species distributed throughout their compact mass. Basalt, 

 and some other allied rocks, must be considered as most intimate mix- 

 tures of Feldspar with Hornblende or Pyroxene, or with both these spe- 

 cies, the individuals being so small as to be no longer recognizable. In 

 several of these rocks which contain Feldspar as one of their ingredients, 

 larger masses of it frequently form concretions separated from the rest, 

 and assume the shape of more or less extended beds. If these be de- 

 composed by the action of the atmosphere, and their situations be favora- 

 ble, Porcelain earth is formed, among the most remarkable of which we 

 notice those in gneiss, at Aue near Schneeberg in Saxojiy, and at Haf- 

 nerzell in the district of Passau. At Carclaise and Cligga in Cornwall, 

 the porcelain earth originates in the decomposition of granitic rocks. It 

 occurs frequently in beds along with ores of iron and titanium, with sev- 



