PHYSIOGRAPHY. CLASS n. 



compact variety perhaps of another species, the prevailing 

 mass of Weissstein. Basalt, and some other rocks, allied 

 with it, as wacke and graustein, must be considered as 

 most intimate mixtures of prismatic Feld-spar or Albite with 

 hemi-prismatic or with paratpmous Augite-spar, or with 

 both these species, the individuals being so small as to be 

 no longer recognizable. The two first have on that account 

 been considered by mineralogists as particular species. In 

 several of these rocks, which contain prismatic Feld-spar as 

 one of their constituents, larger masses of it frequently 

 form concretions separated from the rest, and assume the 

 shape of more or less extended irregular beds. If these 

 be decomposed by the action of the atmosphere, and their 

 situation be favourable, Porcelain earth is formed, among 

 the most remarkable of which we notice those in gneiss, at 

 Aue near Schneeberg in Saxony, and at Hafnerzell in the 

 district of Passau. At Carclaise and Cligga in Cornwall, 

 the porcelain earth originates in the decomposition of gra- 

 nitic rocks. Common Felspar frequently occurs in beds 

 along with ores of iron and titanium, with several species 

 of the genera Augite-spar and Garnet, &c., but it may be 

 rightly considered as a rarity in veins, except in those 

 which are composed of the same species of which the rocks 

 consist which they traverse. In these its varieties are ac- 

 companied by prismatic Axinite, rhombohedral Quartz, 

 several ores of titanium, by rhombohedral Lime-haloide, 

 and other species, and have their surface sometimes covered 

 with scaly particles of prismatic Talc-mica. Sometimes 

 hemi-prismatic crystals are covered with a tetarto-prismatic 

 coat of another species, as in the varieties from Eaveno, 

 which have Albite disposed in parallel position upon some of 

 their faces, particularly upon Pr + co. Labradorite occurs 

 in syenitic rocks, also as a regular constituent in several 

 kinds of gabbro rocks, with hemi-prismatic Schiller-spar and 

 Serpentine. Those crystals which give a porphyritic ap- 

 pearance to the trachyte from the Drachenfels on the llhine, 

 are considered as a variety of glassy Felspar. 



