of which the mountain is formed is
basalt or dolerite. The volcano of Rangitoto in Auckland, New Zealand,
appears to belong to this class.

Basalt is the most fusible of volcanic rocks, owing to the augite and
magnetite it contains, so that it spreads out with a very slight slope
when highly fused. Trachyte, on the other hand, is the least fusible
owing to the presence of orthoclase felspar, or quartz; so that the
volcanic domes formed of this material stand at a higher angle from the
horizon than those of basaltic cones.

[1] _Scenery and Geology of Scotland_ (1865), p. 214.

[2] Humboldt says: "The form of isolated conical mountains, as those of
Vesuvius, Etna, the Peak of Teneriffe, Tunguagua, and Cotopaxi, is
certainly the shape most commonly observed in volcanoes all over the
globe."--_Views of Nature_, translated by E. C. Otte and H. G. Bohn
(1850).

[3] It is supposed that after the disastrous explosion of Krakatoa in
1883 the fine dust carried into the higher regions of the atmosphere was
carried round almost the entire globe, and remained suspended for a
lengthened period, as described in a future page.

[4] Another remarkable case is mentioned and figured by Judd, where one
of the Lipari Isles, composed of pumice and rising out of the
Mediterranean, has been breached by a lava-stream of obsidian.--_Loc.
cit._, p. 123.

[5] Reyer has produced such dome-shaped masses by forcing a quantity of
plaster of Paris in a pasty condition up through an orifice in a board;
referred to by Judd, _loc. cit._, p. 125.

[6] Whymper determined the height to be 20,498 feet; Reiss and Stuebel
make it 20,703 feet. Whymper thinks there may be a crater concealed
beneath the dome of snow.--_Travels amongst the Great Andes of the
Equator_, by Edward Whymper (1892).

[7] Whymper states that there is a prevalent idea that Cotopaxi and a
volcano called Sangai act as safety-valves to each other. Sangai reaches
an elevation (according to Reiss and Stuebel) of 17,464 feet, and sends
intermittent jets of steam high into the air, spreading out into vast
cumulus clouds, which float away southwards, and ultimately
disappear.--_Ibid._, p. 73.

CHAPTER III.

LINES AND GROUPS OF ACTIVE VOLCANIC VENTS.

The globe is girdled by a chain of volcanic mountains in a state of
greater or less activity, which may perhaps be considered a girdle of
safety for the whole world, through which the masses of molten matter in
a state of high