PHYSIOGRAPHY. 



Feldspar Fergusonite. 



205 



The composition of Mikroclinous Felsite is, according to KLAP- 

 ROTH, 



Silica .... 65-00 



Alumina - - - - 20-00 



Potash .... 12-25 



Oxide of iron - 1-25 



Magnesia - - - a trace. 



Water - 0-50 



99-00 



P on g' 

 g on h 1 



ii. Murchisonite. LEVY. 



90 00' 



- ' - 90 



The two cleavages which are at right angles, are like Feld- 

 spar ; the third has a nacreous lustre, and is as easily obtained as 

 the others. The plane P of this figure is not afforded by cleav- 

 age in common Feldspar. 



Color white, with a tinge of red. Opake. 



It occurs at Heavitree near Exeter, forming a compact rock, associa- 

 ted with Quartz, Mica and black Tourmaline ; and is connected with the 

 red marl. 



ih\ Ryakolite. ROSE? 



The experiments of G. ROSE upon the glassy Feldspar, are said to 

 have rendered it apparent that its angles are different from those of Feld- 

 spar, and from the above enumerated varieties, proposed as species. Its 

 sp. gr. = 2-576. It occurs in the lava of Vesuvius, and in that of the 

 Lacher Lake. 



FERGUSONITE. 



Primary form. Right square prism. 



18 



