s of 1000 to 2000 fathoms. It is unnecessary here to
attempt to enumerate all these islands which rise in solitary grandeur
from the surface of the ocean, and are the scenes of volcanic
operations; a few may, however, be enumerated.

[Illustration: Fig. 4.--The Peak of Teneriffe (Pic de Teyde) as seen
from the ocean.--(From a photograph.)]

(_a._) _Iceland._--In the Atlantic, Iceland first claims notice, owing
to the magnitude and number of its active vents and the variety of the
accompanying phenomena, especially the geysers. As Lyell has
observed,[1] with the exception of Etna and Vesuvius, the most complete
chronological records of a series of eruptions in existence are those
of Iceland, which come down from the ninth century of our era, and which
go to show that since the twelfth century there has never been an
interval of more than forty years without either an eruption or a great
earthquake. So intense is the volcanic energy in this island that some
of the eruptions of Hecla have lasted six years without cessation.
Earthquakes have often shaken the whole island at once, causing great
changes in the interior, such as the sinking down of hills, the rending
of mountains, the desertion by rivers of their channels, and the
appearance of new lakes. New islands have often been thrown up near the
coast, while others have disappeared. In the intervals between the
eruptions, innumerable hot springs afford vent to the subterranean heat,
and solfataras discharge copious streams of inflammable matter. The
volcanoes in different parts of the island are observed, like those of
the Phlegraean Fields, to be in activity by turns, one vent serving for a
time as a safety-valve for the others. The most memorable eruption of
recent years was that of Skaptar Jokul in 1783, when a new island was
thrown up, and two torrents of lava issued forth, one 45 and the other
50 miles in length, and which, according to the estimate of Professor
Bischoff, contained matter surpassing in magnitude the bulk of Mont
Blanc. One of these streams filled up a large lake, and, entering the
channel of the Skapta, completely dried up the river. The volcanoes of
Iceland may be considered as safety-valves to the region in which lie
the British Isles.

(_b._) _The Azores, Canary, and Cape de Verde Groups._--This group of
volcanic isles rises from deep Atlantic waters north of the Equator,
and the vents of eruption are partially active, partially dormant, or
ex