The depth
between the islands does not seem to be known, but the 1,000 fathom line
encloses the whole group pretty closely, while a depth of about 1,800
fathoms is reached within 300 miles in all directions. These great depths
render it in the highest degree improbable that the Azores have ever been
united with the European continent; while their being wholly volcanic is
equally opposed to the view of their having formed part of an extensive
Atlantis including Madeira and the Canaries. The only exception to their
volcanic structure is the occurrence in one small island only (Santa Maria)
of some marine deposits of Upper Miocene age--a fact which proves some
alterations of level, and perhaps a greater extension of this island at
some former period, but in no way indicates a former union of the islands,
or any greater extension of the whole group. It proves, however, that the
group is of considerable antiquity, since it must date back to Miocene
times; and this fact may be of importance in considering the origin and
peculiar features of the fauna and flora. It thus appears that in all
physical features the Azores correspond strictly with our physical
definition of "oceanic islands," while their great distance {248} from any
other land, and the depth of the ocean around them, make them typical
examples of the class. We should therefore expect them to be equally
typical in their fauna and flora; and this is the case as regards the most
important characteristics, although in some points of detail they present
exceptional phenomena.

[Illustration: OUTLINE MAP OF THE AZORES.]

NOTE.--
The light tint shows where the sea is less than 1,000 fathoms deep.
The dark tint " " " more than 1,000 fathoms deep.
The figures show depths in fathoms.

_Chief Zoological Features of the Azores._[50]--The great feature of
oceanic islands--the absence of all indigenous land-mammalia and
amphibia--is well shown in this {249} group; and it is even carried
further, so as to include all terrestrial vertebrata, there being no snake,
lizard, frog, or fresh-water fish, although the islands are sufficiently
extensive, possess a mild and equable climate, and are in every way adapted
to support all these groups. On the other hand, flying creatures, as birds
and insects, are abundant; and there is also one flying mammal--a small
European bat. It is true that rabbits, weasels, rats and mice, and a small
lizard peculiar to Made