UNSTRATIF1ED OR IGNEOUS ROCKS. 215 



Basalt is the type of the basics. It is a very dark, 

 almost black, heavy rock, scarcely visibly grained to the 

 naked eye, and breaking with conchoidal fracture. It 

 consists of plagioclase with augite, olivine, and magnetite. 

 Dolerite has a similar composition, but more distinctly 

 crystalline texture, and therefore dark-grayish color. 

 Tachylite is the glassy variety, which, if vesicular, be- 

 comes black scoria. 



The following table is a condensed statement of the 

 composition of the principal kinds of rocks numbered 

 above. The sign x x indicates crystals. 



IGNEOUS ROCKS. 



Two Modes of Eruption. There are two modes of 

 eruption. In the one, the fused mass comes up through 

 chimneys, and flows off in streams (or ejected as cinders 

 and ashes) ; in the other, it comes up through great 

 fissures often hundreds of miles long, and spreads as 



