212 



CLASSIFICATION OF IGNEOUS ROCKS. 



occurs in grains, in this and certain other volcanic rocks, is probably 

 not entirely unconnected with the occurrence of quartz in separate 

 grains in the rock which was metamorphosed ; while, in some cases, 

 we attribute the formation of glass inclusions to the action of heat 

 upon the fluid inclusions in quartz grains, by which the water and 

 alkaline chloride contents have fused the silica with which they were 

 in contact, so as to convert the fluid cavity into a glass " inclusion." 

 If rhyolites are typically the lavas of granites, and do not form a 

 distinct and more highly silicious group, it is because of the diver- 

 sity of rocks included under the name granite ; and just as certain 

 sandstones graduate into certain clays, so certain rhyolites repre- 

 sent some highly acidic granites. 



Zirkel's Classification of Igneous Rocks. 1 Zirkel has always 

 recognised the tertiary lavas as representative in time of the older 

 lavas, and as connected with plutonic rocks. His classification of 

 these rocks, according to the predominant or typical mineral of the 

 felspar family, expresses chemical as w r ell as mineralogical differences 

 between the chief groups, and the affinities which he believes to 

 exist between the older and newer rocks. 



With Quartz or excess of 

 Silica. 



Granite. 



Granite Porphyry. 



Felsite Porphyry. 



Rhyolite. 



Obsidian. 



Perlite. 



Pumice. 



Pitchstone. 



With Hornblende. 



Quartz-Diorite. 



Diorite. 



Porphyrite. 



Hornblende-Porphyrite. 



Quartz-Propylite. 



Propylite. 



Dacite. 



Hornblende - Andesite. 



/. Orthoclase Rocks. 



Without Quartz, with Plagio- 

 clase. 



Syenite. 

 Augite-Syenite. 

 Quartzless Orthoclase 

 Porphyry. 



Trachyte. 



Augite-Trachyte. 



//. Playioclase Rocks. 



With Augite. 

 Diabase. 



Augite-Porphyry. 

 Melaphyre. 



Augite-Andesite. 



Felspar-Basalt. 



Tachylyte. 



Without Quartz, with Nephe- 

 liue or Leucite. 



Foyaite. 



Miascite. 



Orth oclase -Porphyry. 



Phonolite. 



Leucite rock. 



Sanidine rock. 



With Mica. 

 Mica-Diorite. 



With Diallage. 

 Gabbro. 



With Hypersthene. 

 Hypersthenite. 



With Olivine. 

 Serpentine (Forellenstein). 



777. Nepheline Rocks. 

 Nephelinite. 

 Nepheline Basalt. 



IV. Leucite Rocks. 

 Sanidine Leucite Rocks. 

 Leucite Basalt. 



The Basic Series of Volcanic Rocks may be similarly paralleled 

 by analyses of shales, clays, slates, and schists, see p. 225 and p. 285 ; 



1 U. S. Geol. Explor. Fortieth Parallel, vol. vi. p. 6, 1876; compare also 

 Zirkel, "Lerhbuch der Petrographie," vol. i. p. 450, 1866. 



