224 THE SYENITE GROUP OF ROCKS. 



are similarly combined, and both are sometimes embedded in the nephe- 

 line. The biotite is brown. When muscovite occurs it is always 

 associated with the felspar and nepheline, and is never combined with 

 the biotite. Titanite is characteristic, and always has a delicate 

 yellow colour. This rock presents a singular chemical correspondence 

 with phonolite, of which it may be regarded as the plutonic represen- 

 tative. Its composition is as follows : 



Analysis of Foyaite. 1 



Silica 56-23 



Sesquioxide of iron . '17 



Protoxide of iron . . 6' 21 



Alumina 2 3' 1 S 



Lime 2*39 



Magnesia .... -40 



Soda 3-84 



Potassa 5-33 



Titanic acid ... '27 



Phosphoric acid . . '13 



Sulphuric acid ... -09 



Chlorine ... . '07 



Water ro6 



Zircon-Syenite. This rock is a granular compound of orthoclase, 

 elseolite, and zircon, with a little hornblende. It is excellently seen 

 in the south of Norway, between the entrance to the Christiania 

 Fjord and Langesand Fjord, stretching north to Skunsfjeld, south of 

 Konigsberg. Another mass occurs north of Christiania, in the island 

 of Seiland in West Finnmark, and at Cape Comfort and Kittiksut in 

 Greenland. 



The orthoclase is large and shows a silvery play of colour, which 

 may be blue, green, yellow, or red. There is some difference of 

 opinion as to the identification of the felspar. The elseolite has a 

 greasy aspect ; the hornblende is black and rich in soda ; quartz is 

 rare, and zircon is found in long columnar crystals. The rock in 

 Maridal contains 66-39 P er cent, of silica and 1379 per cent, of 

 alumina. Fifty accessory minerals have been described from the 

 Norwegian zircon-syenite, and occur chiefly on the margins of the rock 

 as contact minerals, though many are scattered through it. 



Miascite. Miascite is a large-grained granite-like compound of 

 orthoclase, elseolite, and biotite, differing from zircon-syenite in the 

 substitution of black mica for zircon. The felspar is white or grey ; 

 the elaeolite has the aspect of quartz ; and the black mica, in thin 

 plates, is dark green. Sodalite is frequently met with. Hence this 

 rock approaches to some of the nepheline and nosean-bearing phono- 

 lites. Octahedral zircon is sometimes present in miascite as an 

 accessory mineral. Ilmenite occurs in crystals some inches in 

 diameter. 



This rock is chiefly known from Miask in the Ilmengebirge to 

 the east of the Ural, whence it extends north between granite on the 

 east and gneiss on the west. 



A variety which is rich in soda, and contains hornblende and 

 mica, is termed ditroite, from Ditro in Transylvania. 



1 Dr. C. P. Sheibner, Q. J. G. S., vol. xxxv. p. 42. 



