208 



UNSTRATIFIED OR IGNEOUS ROCKS. 



is then called nevadite. Such granitoid rhyolite may be easily distin- 

 guished from true granite by the presence of the glassy paste. 



Phonolite is a light-grayish crypto-crystalline feldspathic rock, 

 breaking or jointing in very characteristic thin tile-like slabs, which 

 ring under the hammer (hence the name). It consists mainly of orthio 

 feldspar (sanidin and nephelin). 



In the basic sub-group the most common and typical is basalt. 

 This is a very dark, almost black, crypto-crystalline rock, breaking 

 with a dull, conchoidal fracture, and consisting essentially of micro- 

 scopic crystals of plagioclase, augite, and olivin, in a glassy ground-mass 

 of the same. Magnetite is also usually an abundant constituent. Dol- 

 erite has a somewhat similar composition, but lacks the olivin, and is 

 more crystalline in structure, and therefore dark-grayish in appearance. 

 Andesite is a dark-grayish rock, consisting essentially of plagioclase, 

 with hornblende or augite. It is somewhat similar in color to dolerite, 

 but is crypto-crystalline, like basalt, and often roughish to the feel, 

 like trachyte. It has, therefore, been sometimes called trachy-dolerite. 



All the rocks of both these sub-groups, but especially the more 

 typical, have their scoriaceous and glassy varieties. These are the 

 pumices and light-colored scoriae and obsidians on the one hand, and 

 the black scoriae and tachylite on the other. 



The following table is a condensed statement of the composition 

 of the principal kinds in both primary groups, including also intrusives. 

 The sign x x indicates crystals : 



IGNEOUS KOCKS. 



2 3 J .s o 



>& 



. I « o 



B 1° 



r 



£ I \%h 



Rhyolite. 



Vitreous 



ground-mass. 



( Quartz, 

 c < Orthoclase 

 ( (sanidin). 



Trachyte. 

 Vitreous 



ground-mass. 



j Orthoclase 

 J (sanidin) 



Phonolite. 



Vitreous 



ground-mass. 



\ Sanidin, 

 * 1 Nephelin. 



Quartz-porphyry. 

 Micro x x ground -mass. 

 + 

 ( Orthoclase, 

 x * 'I Quartz, 



Felsite. 



Micro j Orthoclase, 

 x x "J Quartz. 



Granite. 

 { Quartz, 

 x ■< Orthoclase, 

 f Mica. 



Syenite. 



j Orthoclase. 

 j Hornblende. 



Andesite. Basalt. 



Vitreous Vitreous 



ground- mass. ground-mass. 



+ + 



Plagioclase, ( Plagioclase, 

 Augite, or > 

 Hornblende. 



{Plagiocli 

 Augite, 

 Olivin. 



Diorite. 



Same as 



below, 



but micro 



Diabase. 



Same as 



below, 



but micro 



Diorite. 



j Plagioclase. , 

 ( Hornblende' 



Gaobro. 

 Plagioclase, 

 Augite, 

 Olivin. 



Modes of Eruption. — There have been in geological times two gen- 

 eral modes of eruption. In the one the lavas have come up through 



