IGNEOUS AGENCIES. 139 



solidated either on the spot or after transportation and 

 sorting, is called tufa. 



Physical Conditions of Lava. If lava cools very 

 slowly, the minerals of which it is composed separate and 

 crystallize more or less perfectly. This is stony lava. If 

 it cools rapidly, it forms volcanic glass. If the volcanic 

 glass be full of vapor-bubbles, it forms scoria. If volcanic 

 ashes mixed with water solidifies, it makes tufa. Thus 

 there are four physical states in which we find lava, viz., 

 stony, glassy, scoriaceous, and tufaceous. 



Classification of Hardened Lavas. Hardened lava 

 consists essentially of two principal minerals, viz., feld- 

 spar and augite. * If the former predominate, it is called 

 feldspathic ; if the latter, augitic lava. These two min- 

 erals are often not detectable except with the microscope, 

 and yet the two kinds of lavas may usually be distin- 

 guished by the eye. The lighter colored and lighter 

 weighted are usually feldspathic ; the darker and heavier, 

 augitic. The feldspathic lavas are said to be acidic ; the 

 augitic, basic. Both of these kinds take on the four 

 physical states mentioned above. Feldspathic lava, in 

 the stony condition, is trachyte ; in glassy condition, 

 obsidian ; in scoriaceous condition, pumice ; in tufa- 

 ceous, the light-colored tufas. Augitic lava, if stony, is 

 basalt ; if glassy, pitchstone ; if scoriaceous and tufa- 

 ceous, black scoricB and tufas. 



Gases and Vapors. The gases ejected from vol- 

 canoes are steam, chlorhydric acid, sulphurous acid, 

 sulphhydric acid, and carbonic acid (H 2 0, HC1, S0 2 , 

 H a S, C0 2 ). The first three are characteristic of true 

 eruptions, the others of feeble, secondary volcanic activ- 

 ity. Of all, steam is by far the most abundant. In vol- 

 canoes of the explosive type the quantity of steam is 

 often enormous. This fact strongly suggests this vapor 

 as the main agent of eruption. Flames are often spoken 

 * The pupil ought to be shown specimens of these minerals. 





