302 



PHYSICAL GEOLOGY 



tend to collect into crystals. When the process is long continued 

 the point of saturation of the other minerals is reached, crystals are 

 formed, and a rock composed entirely of crystals results. Granite 

 and coarse-grained traps (p. 330) are such rocks. If the cooling is 

 more rapid, rocks composed of fine crystals such as rhyolites and 

 basalts (p. 331) may be formed. When, however, a lava flow cools 

 so rapidly that no crystals or only a few can form, volcanic glass, or 

 obsidian, is produced. Often a lava flow passes from a glassy to a 

 crystalline state from the surface downward. When such cooling 

 lava is under little pressure, the gases in the surface portions are able 

 to expand, and often produce a surface which is called scoriaceous 

 (Fig. 294) if cindery, or pumiceous if the pores are very numerous 

 and small. 



Types of Volcanoes 



Because of their destructiveness volcanoes probably inspire greater 

 interest than any other natural phenomenon, and it will consequently 

 be well to discuss briefly the various types of volcanoes. It should, 

 however, be remembered that the aggregate work of volcanoes is 

 inconsiderable as compared with that of streams, the ocean, and other 

 less conspicuous forces. 



The chemical composition of lavas, as will be seen, has a con- 

 siderable influence upon the character of eruptions, but the principal 

 factor is the physical state of the lava; i.e., whether it is fluid or 

 viscous and stiff. If the molten rock is so liquid that the gases can 

 escape rapidly, they do not accumulate into great bubbles which 

 throw the lava high into the air when they break. If on the other 

 hand the lava is stiff", the gases gather into great bubbles which upon 

 bursting throw out the lava as dust, cinders, and bombs. 



/. The Explosive or Vesuvian Type 



(1) Vesuvius. — Vesuvius has been more carefully studied than any 

 other volcano in the world and is yearly ascended by so many travelers 

 that its value as an illustration is unsurpassed. 



Previous to 79 a.d. Vesuvius seemed to be extinct, but before the 

 close of that year a great eruption occurred which destroyed the cities 

 of Herculaneum and Pompeii, and laid waste a great extent of coun- 

 try. During this eruption no lava was poured out, but a large part 

 of the crater was blown off and the outline of the mountain greatly 

 changed. Ash and dust were thrown to great heights and were carried 



