33 TRACHYTIC ROCKS. 



The Family of Quartz-less Orthoclase Roclcs. 



Syenite might be grouped with the granites which it resembles in 

 external appearance, because the conditions of consolidation were the 

 same. But as it does not contain quartz, it is evident that the strata 

 which were metamorphosed to form it must originally have been dis- 

 similar. Typically, syenite contains hornblende ; but occasionally the 

 hornblende is replaced by augite, or in Calabria by mica. The mica- 

 syenite of Kosenbusch is the minette or mica-trap of authors. Professor 

 Bonney regards mica-trap as a family in which he places minette-ker- 

 santite, minette-felsite, mica-diorite, and kersantite-porphyrite. Augite- 

 syenite, like mica-syenite, is chiefly known in dykes. The volcanic 

 representative of hornblende-syenite is beyond doubt trachyte ; though 

 trachyte, like rhyolite, contains glassy felspar, instead of the opaque 

 orthoclase of the plutonic rocks. 



Trachyte differs from the granite series in the circumstance that 

 most varieties of the rock do not contain crystals of quartz ; or when 

 quartz is present, it is in a form named tridymite. The percentage of 

 silica in the rock does not fall much, if at all, below that of granite ; 

 and the trachytes are closely related to the granites. In North America 

 quartz-trachytes are recognised. Trachytes properly so called are 

 rough crystalline rocks ; they occur as lava streams, but sometimes the 

 crystalline trachytes form mountain masses. 



These rocks are probably of all geological ages, though the true 

 trachytes have hitherto only been met with in the newer rocks, being 

 represented in the older series by quartzless felsites. We group 

 phonolite with syenite, because it bears much the same relation 

 to nepheline-syenite that felsite and rhyolite hold to granite. 



Syenite 



Zircon- 

 syenite 



Porphyrite 



Sanidine 

 trachyte 



COMPOSITION. 



Granular crystalline mixture of 

 orthoclase and some oligoclase, 

 with hornblende and magnetite. 

 Many syenites contain mica, 

 quartz, nepheline, or augite. 



Granular crystalline mixture of 

 orthoclase, nepheline, zircon, and 

 a little hornblende. 



A dark felsitic matrix, chiefly 

 of oligoclase or orthoclase, with 

 crystals of felspar, forming fel- 

 spar porphyry, or mica forming 

 mica porphyry, or hornblende 

 forming hornblende porphyry. 



I Granular or compact matrix of 



j sanidine, with crystals of sanidine, 



hornblende, and magnesia mica. 



i 



LOCALITIES. 



Charnwood Forest, Dresden, 

 Guernsey. 



Laurvig and Brevig in Nor- 

 way. 



Elfdalen in Sweden, Lenne- 

 gebiet in the Harz, often 

 with columnar joints, Thur- 

 ingian Forest, Pentland 

 Hills, near Edinburgh. 



Lavas of Monte Nuovo, Berk- 

 urn, near Bonn; Koberts- 

 hausen, in Hesse Darm- 

 stadt. 



