HANDHOOK FOR THE DEPARTMENT OF GEOLOGY. 577 



Structure. — In structure tbe trachytes are rarely granular, but pos. 

 sess a fine, scaly or microfelsitic groundmass, rendered porphyritic 

 through the development of scattering crystals of sanidin, 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 microscopic structure of the trachyte of Monte Vetta is shown in 

 Fig. 5, Plate cxx, as prepared from specimen 36332. 



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



Classification and nomenclature. — They are divided into hornblende, 

 augite or mica-trachytes, according as any one of these minerals pre- 

 dominates. The name sanidin-oliyoclase trachyte is sometimes given to 

 trachytes in which both these feldspars appear as prominent constitu- 

 ents. (See specimens from the Siebengebirge, Prussia.) The presence 

 of quartz gives rise to the variety quartz-trachytes. (See under rhyo- 

 lites.) The glassy form of trachyte is commonly known under the 

 name of the trachyte pitchstone (specimens 36272, 36276, and 38786 and 

 72995 from Hungary, Isle of Ponza, and the Isle of Ischia), or if with 

 a perlitic structure simply as perlite. (Specimens 36277, 36278 and 

 36279 from Hungary.) In his most recent work Professor Eosenbusch 

 has included the glassy forms under the name of hyalo -trachyte. 



The following localities and varieties are represented. 



Trachyte: Game Ridge, near Rosita, Silver Cliff Region, Colorado, 70605 ; uear Car- 

 bonate Camp, Black Hills, Dakota, 159097 ; Me of Ischia, 36320 to 36326, inclu- 

 sive, 38336, 35857, and 72,994 to 72998, inclusive ; Euganaen Hills, Italy, 36318 

 and 36319 ; near Naples, Italy, 36314, 73013, 73014, and 73017 ; Monte Verginio, 

 Rome, Italy, 36328 ; Astroni, Naples, Italy, 36327 ; Monte Bracalon, Italy, 34792 ; 

 Monte Nuovo, Naples, Italy, 36329; Monte Somma, Italy, 36330 and 36331 ; Laa- 

 cher See, Prussia, 36315, 36316, and 36317 ; Isle of Ponza, Italy, 38788 and 73004 ; 

 Isle of St. Stefano, 73007 ; Visegrad, Apatkuter, Hungary, 34569 ; Moscar, Hun- 

 gary (Biotite Hypersthene Trachyte), 70183 ; Hlinik, Hungary (pumiceous), 

 36312; the Siebengebirge, Prussia, 34644, 36305 to 36308, inclusive, and 36348; 

 Hulsberg, Nassau, Germany (Sanidin oligoclase-trachyte), 36310; Inselberg, 

 Prussia (sanidin-oligoclase-trachyte), 36311. 



Hyalo trachyte — trachyte pitchstone. — Hlinik. Hungary, 36272; Kozelnik, Hungary, 

 36376 ; Isle of Ponza, 38786; Pusti Hrad, Hlinik and Kremnitz, Hungary (Perlitic 

 varieties), 36277, 36278, and 36379. 



5. The phonolites. 



Phouolite, from the Greek word <pujvr h sound, and Xiftoq, stone, in allu- 

 sion to the clear ringing or clinking sound which slabs of the stone emit 

 when struck with a hammer ; frequently called clinlcstone for the same 

 reasons. 



Mineral composition. — The phonolites consist essentially of sanidin 



and nepheline or leucite, together with one or more minerals of the 



augite hornblende group, and generally hauyn or nosean. The common 



accessories are plagiociase, apatite, sphene, mica, and magnetite; more 



H. Mis. 129, pt. 2 37 



