362 



Leucite and nepheline-bearing rocks are not so common as felspar-bearing 

 rocks. Elseolite-syenites occur in southern Norway, in South Greenland 

 (eudialyte-bearing elseolite-syenite), in the Portuguese Province of Algarve, 

 at Ditro in Hungary, at Miask in the Ilmen mountains of Russia, in the 

 north-west of New Jersey, at Lichfield in Maine, in the Cape Verde Islands, 

 off the west coast of Senegambia (Los Islands, and the island of Tumbo) 

 and in Brazil. Phonolite is recorded from Saxony, Bohemia, the Eifel, 

 the Westerwald, South Thuringia, the Vogelsgebirge, the Rhongebirge, the 

 Kaiserstuhl and Hegau in Baden, Auvergne, Sardinia, Tripoli, Kordofan, 

 Cape Verde and Canary Islands, Aden, Fernando Noronha and the main- 

 land of Brazil. The phoiiolite of the Wolf rock has already been 

 mentioned. 



Leucite rocks occur in the Eifel, in the neighbourhood of the Laacher 

 Lee, in the Westerwald, Westphalia, the Vogelsgebirge, Baden (Kaiserstuhl), 

 Saxony, Bohemia, Scania (S. Sweden), Italy (including the lavas of 

 Vesuvius and Monte Somma), Sardinia, Cape Verde Islands, north-west 

 Persia. Java, and Wyoming in U.S.A. Nepheline rocks, other than phono- 

 lites, occur in the Eifel, the Westerwald, the Taunus, Hesse, the Odenwald, 

 Baden, the Rauhe Alp, the Vogelsgebirge, the Rhongebirge, Thuringia, the 

 Fichtelgebirge, Saxony, Bohemia, Silesia, Spain, South Greenland, South 

 Sweden, the Cape Verde and Canary Islands, Montana U.S.A. and the 

 Sandwich Islands. It will be observed that the leucite and nepheline 

 rocks generally hang together so far as distribution is concerned. 



DESCRIPTION OF THE MINERALS. 



Orthoclase. The orthoclase of the elceolite-syenites generally occurs 

 in allotriomorphic grains of nearly equal dimensions in the different 

 directions. When, as sometimes happens, the elseolite-syenite assumes 

 more or less the structure of a phonolite the orthoclase may become 

 tabular in consequence of the development of the clino-pinacoid. In 

 the elseolite-syenites of southern Norway the orthoclase is rich in 

 soda (soda-orthoclase of BROGGER) and often becomes idiom orphic with 

 development of the forms T, 1 and y. Felspars of this form are 

 characterized by frequently giving rhombic sections. The orthoclase 

 often shows micro-perthitic intergrowths with a triclinic felspar. 



In the rocks with trachytic texture (phonolites) the orthoclase is 

 present in the form of sanidine. It occurs as. porphyritic crystals and 

 frequently also as a constituent of the ground-mass. The crystals are 

 usually columnar in form and elongated in the direction of the 

 clino-diagonal axis. Twinning on the Carlsbad plan is sometimes 

 present, but does not appear to be so common as in the sanidine- 

 trachytes and granites. The columnar crystals of sanidine are fre- 

 quently arranged in a parallel manner in the rock, and when this is 

 the case the mass possesses a marked fissility. 



It is sometimes difficult to distinguish between the small sanidines 

 of the ground-mass and nepheline. The best way of discriminating 



