92 IGNEOUS AGENCIES. 



It is evident that, of the two kinds of eruption mentioned above, 

 the quiet type is characterized by igneous fnsion, the explosive type by 

 aqueo-igneous fusion. In the former the heat is great, but the amount 

 of water is small ; while in the latter the heat is less, but the amount 

 of water far greater. 



The rapidity of the flow of a lava-stream depends on its fluidity. In 

 the Hawaiian volcanoes the lava, where it issues from the crater, has 

 been seen to flow with a velocity of fifteen miles an hour; while 

 Yesuvian lava seldom flows at a rate of more than two or three miles 

 an hour. Lava, like glass, passes through various grades of viscous 

 fluidity in cooling. It gradually becomes so stiff that it may flow only 

 a few feet per day. The froth or scum which covers the surface of a 

 lava-stream quickly cools and hardens into a crust of vesicular lava, 

 which may even be walked upon while the interior is still flowing 

 beneath. In this way are often formed long galleries. Also the run- 

 ning together of the contained gas-bubbles and steam-bubbles forms 

 huge blisters in the viscous mass, which, on hardening, form cavities 

 often of great size. Thus, recent lavas have often a cavernous and 

 galleried structure (page 76). 



Classification of Lavas. — Miner -((logically, lava consists essentially 

 of feldspar, augite, and magnetite, either intimately mixed, as in glassy 

 lava, or aggregated in more or less distinct particles or crystals, as in the 

 stony varieties. Now, feldspar is a light-colored mineral, having a spe- 

 cific gravity of about 2 "5, while augite and magnetite are usually very 

 dark-colored minerals, having specific gravities of about 3*5 to 5. It 

 is evident, therefore, that in proportion as feldspar predominates, the 

 lava is lighter colored and of less specific gravity ; and in proportion 

 as augite and magnetite predominate, the rock is darker and heavier. 

 Chemically, feldspar is a silicate of alumina and alkali, with excess of 

 silica (acid silicate). The alkali may be either potash, and then it is 

 called potash feldspar, or orthoclase ; or else it is soda and lime, and 

 then it is called soda-lime feldspar, or plagioclase. Of these two the 

 former is the more acid. Augite is a silicate of lime, magnesia, and 

 iron, with excess of base (basic silicate). Therefore, lava may be di- 

 vided into two classes — acidic lavas and basic lavas. In the former, 

 feldspar predominates, in the latter augite. Moreover, in the one the 

 form of feldspar is orthoclase, in the other plagioclase. Further, it is 

 seen that all lavas are multiple silicates, like glass : they are, therefore, 

 true glass-mixtures. Now, the acidic lavas are a more difficultly fusi- 

 ble, the basic lavas a more easily fusible glass-mixture. Either of 

 these two kinds of lava may exist in any of the conditions mentioned 

 above — viz., as stony, glassy, vesicular, or tufaceous lava, Trachyte is 

 an example of acidic lava, and basalt of basic lava in a stony condition. 

 Pumice is a peculiar vesicular variety of feldspathic lava. 



