HEAT — VOLCANOES. 699 



fracturing the cone, and sometimes increasing, sometimes dimi- 

 nishing, its slope), and 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 vertical cascades of solid lava occur about Mount Loa and other volcanoes. 

 As Prevost observed, flowing lava, like flowing beeswax, if stream follows 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°, 20°, and 40° along the stream formed at the erup- 

 tion 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, 2000 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 are 

 small cones over the very region where the large part of the cone has sunk, 

 the fracture must have occurred before the volcano was extinct. 



Mount Somma is part of an outer wall to Vesuvius ; and it is supposed, 

 with good reason, that the fracture of the mountain at an eruption reduced the 

 mountain to its present size. 



The Val del Bove is a famous gorge or valley, with precipitous sides, 1000 to 

 3000 feet high, in the upper slopes of Mount Etna. Fresh-looking lavas cover 

 the bottom, and dikes intersect the sides. It has been regarded as the result 

 of subsidence. It is altogether probable, as suggested by the author in his 

 Report on Volcanoes, that at its head it was once a crater, like Kilauea or the 

 summit-crater of Maui. The conditions within and about the great depression 

 accord with this view. 



5. Subordinate volcanic phenomena. 



1. Solfataras. — Solfataras are areas where sulphur-vapors escape 

 and sulphur-incrustations form. They occur away from intense 

 volcanic action. Incrustations of alum are common in such 



