VOLCANOES. 727 



4. Mixed Cones. — The cones which, like Vesuvius, and many of 

 the great volcanic peaks of western America, as Cotopuxi, Arequipa, 

 etc., are formed partly of lava and partly of cinders or tufa, may have 

 any angle of slope, up to thirty-five degrees. They may be lava be- 

 low, and terminate in a lofty cinder-cone of forty degrees. The crater 

 may be nearly like that of the cinder-cone, — a deep cavity, with the 

 walls thin, compared with those of the simple lava-cone. There is no 

 fixed order in the alternations of lavas and cinder or tufa layers ; the 

 lavas have generally flowed out most freely in the early stages of a 

 volcano. 



3. Volcanic Action. 



1. AGENTS CONCERNED. 



The agents concerned in volcanoes are (1) lava; (2) vapors or 

 gases. 



1. Kinds of Lava and Volcanic Rocks. — The cooled lava of the 

 volcano is usually more or less cellular, because of the expanding 

 vapors in it while it was hot, though sometimes quite free from cel- 

 lules. The more cellular kind is called scoriaceous lava ; or, if very 

 openly cellular, volcanic scoria, or slag. The cinders and ashes (lapilli 

 of the Italians) are fragments of scoria or lava. A layer of the stony 

 lava, when it has flowed from open vents, like those of Kilauea, has 

 sometimes an upper portion, a few inches thick, of scoria, which was 

 like a scum on the liquid stream, and this scoria is often glassy, like 

 the slag of a furnace. 



The kinds of igneous rocks are described on pages 76 to 79. Among 

 these, the most common are : (1) Trachyte and felsyte, acidic rocks, 

 alike in consisting chiefly of orthoclase (potash-feldspar) ; and (2) dol- 

 eryte (of which basalt is a variety), a heavy basic rock, consisting of 

 the iron-bearing mineral augite and the lime-and-soda feldspar labra- 

 dorite, with some magnetite. The latter often contains chrysolite 

 (olivine), and is then called peridotyte. The common Vesuvian lava 

 is a heavy augitic rock like doleryte, but it contains leucite (a potash- 

 yielding mineral, called omphigene in France), in place of labradorite, 

 and it is named amphigenyte. Of these kinds, doleryte is far the most 

 abundant. The felsyte is often porphyritic, or porphyry. Dioryte, 

 andesyte, augite andesyte, syenyte, granite, are among the less com- 

 mon igneous rocks. Andesyte is very common in western America. 



Volcanic glass and pumice (the latter a light feldspathic scoria), oc- 

 cur often with feldspathic lavas. The glass, when microscopically 

 examined in thin slices, is found to contain incipient crystals ; some 

 of them in short prisms (belonites), supposed to be feldspar, and others 

 capillary (trichites), which have been thought to be hornblende; and 



