VOLCANOES. 715 



viscid. In cases like the latter, the escape of vapors would be more 

 likely to be repressed until violent paroxysmal effects became a con- 

 sequence of the accumulation ; and this may be one reason of the 

 earthquakes attending the eruptions of such volcanoes. 



4. Origin of the Forms of Volcanic Cones. 



The general form cf the growing mountain has been stated to de- 

 pend on the nature of the material ejected, whether lava, tufa, or 

 cinders, or combinations of these. But there are modifications arising 

 from other causes. The principal one is the following : — 



The angle of declivity in a growing cone depends on the part of the 

 cone from which the eruptions take place. Overflows at top, if de- 

 scending but part of the way to the base, increase the height and 

 steepness ; but, descending all the way to the base, they add to the 

 magnitude of the cone without varying the general slope. In fissure- 

 eruptions, fissures at the summit widen the top and increase the slope, 

 for it is like driving in a wedge ; but fissures and outflow about the 

 base spread the base and diminish the average slope : the southeastern 

 slope of Mount Loa spreads out for a score of miles, at an angle of 

 one to three degrees, owing to this flattening process. The slope, 

 then, of a cone depends on the concomitant action of the force causing 

 eruption (this force fracturing the cone, and sometimes increasing, 

 sometimes diminishing, its slope), with the ejection of lava or other 

 material over the sides. 



The slope of flowing lava, while generally small and producing cones of small angle, 

 may still be of almost any angle. It forms continuous streams of 30°; and even verti- 

 cal cascades of solid lava occur about Mount Loa and other volcanoes. As Prevost 

 observed, flowing lava, like flowing beeswax, if stream follow stream rather rapidly, 

 and not too copiously, so that one becomes melted to another, may make layers of great 

 thickness, having a large angle of inclination. Hence, while the average angle of a 

 lava-cone is small, because lavas when in a very large outflow spread rapidly and easily, 

 there are many regions of much steeper angle, over its declivities. The author observed 

 a stream descending into the crater of Kilauea, at an angle of 30°. It was, however, 

 hollow, the interior having run out after the crust had formed. Mr. Coan mentions the 

 frequent occurrence of slopes of 15° to 20° and more, along the stream formed at the 

 eruption of Mount Loa in 1855. 



The outflow of lavas from a vent is an undermining process ; and the region about the 

 crater sometimes subsides as a consequence of it. There are many fractures and a large 

 depressed border, around Kilauea, produced by this means. 



The violence attending eruptions, at times, opens widely the mountain, and makes deep 

 gorges, that become filled by lavas. Maui, one of the Sandwich Islands, has a volcanic 

 mountain 10,000 feet high, a crater like Kilauea, at summit, 2,000 feet deep, and two 

 deep valleys with precipitous sides leading down to the coast, one northward and the 

 other eastward, where the lavas flowed off at the last eruption. It seems as if a quarter 

 of the island had been started from its foundations. Oahu consists of parts of two 

 volcanic mountains. The one of them which is most entire is only a remnant of the 

 old cone, — about one-third; a precipice twenty miles long and one to two thousand 

 feet high, the course of a great fracture, is a grand feature of northern Oahu. As there 



