THE NEPHELTNE SYENITES 73 



saniclin, hornblende, augite of black mica. The texture is 

 porous, and the rock possesses a characteristic roughness to 

 the touch; hence the derivation of the name as given above. 

 Perlitic structure is common in the glassy forms. The micro- 

 scopic structure of the trachyte of Monte Vetta is shown in 

 Pig. 5, PL 5. 



Colors. — The prevailing colors are grayish, yellowish, or 

 reddish. 



Classification and Nomenclature. — They are divided into 

 hornblende J augite, or mica trachytes, according as any one of 

 these minerals predominates. The name sanidin-oUgoclase trachyte 

 is sometimes given to trachytes in which both these feldspars ap- 

 pear as prominent constituents. The presence of quartz gives 

 rise to the variety quartz trachytes. (See under rhyolite.) The 

 glassy form of trachyte is commonly known under the name of 

 trachyte pitchstonc, or if with a perlitic structure simply as per- 

 lite. In his most recent work Professor Rosenbusch has included 

 the glassy forms under the name of hyalotrachyte, 



3. THE FOYAITE-PHONOLITE GEOUP 



This group differs from the last mainly in the partial replace- 

 ment of the potash feldspars by the closely related mineral 

 elffiolite or nepheline. In includes therefore those plutonic and 

 effusive rocks commonly known under the name of elmolite or 

 nepheline syenites and the phonolites. In their silica and potash 

 percentages it will be observed they differ not greatly from 

 the syenites proper, but are much more rich in soda and corre- 

 spondingly poor in lime. They may be described in detail as 

 follows : 



(1) THE NEPHELINE (EL-ZEOLITE) SYENITES: EOYAITS 



Nepheline from the Greek v€<fi€X7}, a cloud, since the mineral 

 becomes cloudy on immersion in acid. Blseolite from sXaiov^ oil, 

 in allusion to the greasy lustre. Syenite from Syene in Egypt. 



Mineral Composition. — The essential constituents of this 

 group are nepheline (elgeolite) and orthoclase, with nearly 

 always a pyroxenie or amphibolic mineral and a plagioclase 

 feldspar. The common accessory minerals are sphene, sodalite, 

 cancrinite, zircon, apatite, black mica, ilmenite and magnetite, 

 with occasional leucite, melinophane, and also tourmalines and 



