As I returned, I could not ¡ but remark the beautiful hedges of
rosemary and naturalized pelargonia, even at-a height of 0000 feet
above the sea.
Having no more facts'to offer on the Geology of Madeira, I will
venture to submit some concluding remarks, i The probability, that
Madeira and Porto Santo, from their vicinity to the- Canaries,
belong to the same system of. formation, leads us- to suspect, even
before we have examined them, that they cannot have been
created by amarine volcanos One thing is clear at first sight,
viz., that the. masses of basalt have not pre-existed as rock of a
different nature, and were afterwards heated in situ, and penetrated
by vapours: every appearance indicates, that these masses
have been elevated as fluid, and streamed from the mouth of a
crater. It next occurs to us, that had the island of Madeira been
entirely created by a marine volcano, its base, if not its bulk;
would, probably, (arguing from analogy): foe composed of pumice
and cinders ; both of which are found in comparatively small
quantities, and alternating with basalt and tufa. The discovery
of the vast bed of transition limestone below the basalt, And continuing
to a depth of 700 feet, until its approach to the level of
the sea allows us to trace -it no further, confirms our conclusion;
and: seems to demonstrate, that Madeira pre-existed as a mass
of transition, or probably of primitive and transition rocks, afterwards
rent by a marine volcano, which covered and elevated
the island by successive streams and ejections ¡of basalt and tufa."
to Porta S. Jorge, 12 G. mile;-:, according to Almeida, and 12j according to Johnston,
I make the'circumference by Johnston’s map,'about 96 G. miles.
” Although M. Broussonet’s assertion, that the island of Gomera contains a mass
of granite and mica-slate, remains' unconfirmed, ye't M. Escolar has since found a
block of primitive sienite in Fortaventura, and Baron Von Bijch has found another
primitive roek in Palma.—Humboldt's Relation Historique, Supplement, p. 640.
“ Had the basalt and tufa of Madeira been formed, or deposited, beneath the sur-
I have before remarked, that the ridges of basalt diverge
from the more central heights behind Funchal, descend boldly
to the sea like the gigantic buttresses of some vast interior mountain,
and so distinctly indicate the courses of those igneous
streams which enveloped the island, that they would almost seem
to have been arrested and indurated as they flowed, as an evidence
to future ages. The hills and vallies which existed in the primitive
island, at the time of the basalt first breaking through,
and flowing over it, and the frequent slips of the first deposits
of tufa under the superincumbent weight- of basalt, must have
contributed still more, than the long continued action „of torrents,
to its present appearance, and to the unequal depths of the strata.
The variation in the sections and aspect of the island, seem to
me, to foe explained by the considerations, that there has been
evidently more than one eruption (from the different alternations
and varieties of the basalt, tufa and scoriae) -;' and that, in the second,
streams of basalt must in some places ¡have pursued, from the very
mouth of the crater, and in others have been diverted into, a
different course or direction to that; of the former streams, which
must occasionally have presented themselves as obstacles or barriers0..:
The same reasoning, confirmed by similar evidence,
face of the ocean, and afterwards lifted up, instead of forcing itself through, and flowing
over a pre-existing formation, we should not, I conceive, find such a striking
continuity in the basaltic coulies and ridges; we should be unable to trace them to
that central point in the interior of the island from which they have evidently proceeded;
and the different beds of tufa near the sea, instead of presenting such a
regular appearance, and such a continued horizontal drift line, (Plate 3, A.) would be
generally distorted and confused. Some more recent formation (perhaps a fossil
limestone) would probably be found immediately beneath the basalt at St. Vicente,
instead of the regular bed of transition limestone.
v It appears to me, that the only alternative to this more probable and simple conclusion,
is, to infer, from finding the tufa intersected by the dikes laying above the
compact basalt in the highest parts of the interior of the island, that wherever the