is applicable to the shallow, ribanded beds of tufa by the water
side, which are not likely to have been washed down (by the
excessive rains, or inundations which generally accompany volcanic
eruptions), and deposited precisely in the same quantity or direction,
at different eruptions, and under circumstances, differing
in some, if in no great degree p. The deep beds of tufa in the
interior seem to have been deposited confusedly, just as they were
vomited from the crater; for I cannot reconcile myself to the
opinion, that such vast masses can have resulted from the premature
decomposition of the' scorise, which still remain perfect,
and in distinct layers ; although this tufa may perhaps have been
vomited as detached matter, afterwards agglutinated by rains and
torrents.
The next question is, are there any remains of the grand crater,
and where was it situated? To the former part of the question
I would answer, no; and to the latter, (recalling the directions of
the various streams and ridges of basalt, and their narrowness
and greater, depth in that neighbourhood, from which they all
appear to have taken their departure) between the peaks of Ruivo,
former is found to compose the cliffs by the sea side, the latter (and the various strata
we have described beneath it) must have given way, and sunk beneath the bosom of
the ocean. *
p That there was a considerable interval between the last, and the preceding eruption,
the streams from both of which must have destroyed all vegetation in their course,
seems evident, from the uppermost beds of tufa being found to contain fragments of
wood in different parts of the interior of the island. Of the wood found in the tufa
(200 feet above the sea) near Cani§al, I merely saw a specimen for a moment, and that
in the hand of another person, but it appeared to me to have passed into woodstone
(holzstein, W.). In another specimen brought from the neighbourhood of the ice
house, (upwards of 4000 feet above the sea,) and which will be found amopgst those
sent to the Geological Society, the wood, thickly imbedded in an indurated, compact,
red tufa, is still soft, and comparatively unaltered: it has evidently belonged to full
grown trees, and in its porous nature, and the distance, of its fibres, seems to me to
resemble that of the draccena more than any other,
Grande, and Canarios. That the plateau of the Poul was also
another point of eruption, I have no doubt, for the same reasons ;
but I have already submitted why there has, probably, never been
anything like a crater there. Some have considered the bay
of Funchal to be the segment of a large craterq, but the rocks
of the bay do not afford more evidence of calcination than those of
the interior ; they axe not at all vitreous, or approaching the
nature of obsidian, and instead of rising in lofty masses above the
level of the water, as if they had formed the walls of a crater,
they occur in basaltic strata of inconsiderable depth, alternating
with tufa, and with the most evident indications of having flowed
from the heights in the centre and interior of the island, which
are from 3500 to 6000 feet higher than this pretended crater.
When we recollect how fragile, how easily decomposed and dispersed,
all the parts of a crater (constantly attacked by gases
and vapours) are, compared with the streams which issue from
it ; that nearly one quarter of the cone of Vesuvius fell in a
single eruption, and that during a repose of less than a century
and a half, it became covered with trees and plants—we cannot
wonder that all traces of the grand crater of Madeira should have
been effaced in the many ages which have elapsed since its
creation : the very convulsions which have so evidently rent the
Courais may have undermined its tottering remains. The basaltic
rocks of Madeira are probably of the same age as those of
Teneriffe, and, consequently, considerably older than the lavas
produced by existing causes in the latter island ; causes which
from local circumstances have not extended to Madeira.r
’ M. Guillin, in the Appendix of Bory St. Vincent’s Voyagé. See note to p. 25,
supra.
r Shocks of earthquakes were felt in Madeira in 1813-14, from the N.W. ; and
January 11th, 1816. The latter is said to have lasted from fifteen to twenty minutes,
and to have cracked the beams of the houses, throwing the inhabitants against the
walls ; it was felt at Lisbon and the Azores.