these dikes to the sea, immediately north of PicO ; Juliana,
(apparently the highest in the island, yet not much exceeding that
of Castello) I found a deposit of native alum; and almost on a
level with the sea, another of a bright orange ochre“, accompanied
by a ferruginous spring. The tufa would seem to be strongly
impregnated with muriate of soda, for all the streams which issue
from it, at whatever height, are very brackish, whilst the water
from the sandstone is always pure, even when the sources are
much nearer the #ea than those of the tufa”; I saw' a large piece
of fibrous gypsum at the Governor’s, and it was said to be found
in abundance in the north-eastern rocks, or islets, of Peseada and
Lorenzo, and in a third, about two miles from the shore. I should
have made a point of visiting these rocks ; the rock of Fonte
(about a mile from the northern shore), the island of Farol (about
half a mile from Faehoda, the W.S.W. point of Porto Santo),
which appeared to me to be covered with scoriaceous basalt, overlaying
the tufa, and dipping rapidly to the W.S.W., and thè island
of da Serra about half a mile from Dos Frades, the S.S.E. point of
Porto Santo ;—but to do this, which would not have occupied two
days more, I must have given up the opportunity of returning
with the Genoese, and hired a boat at my own expense, to carry
me back to Madeira, which was out of the question. I was prevented
from examining the Desertas (which have evidently been
divided by the strong south-west current that sets against them),
by similar considerations.
There are no traces of primitive or secondary formations in
This ochre was evidently formed from the decomposition of the basalt., for on
breaking a lump, a nucleus of basalt was found within it. It is a richer pigment than
the ochre imported at Madeira from Teneriffe, to paint the doors and wainscots of the
houses, in imitation of mahogany.
* In this respect, itf approaches Baron de Humboldt’s argile muriatifère of Araya,
near Cumana, also found nearly 13,000 feet above the sea, on the Cordillieres of New
Grenada. 'Relation Historique, 1. 2, c. 5.
Porto Santo ; but the calcareous tufa just described, which is a
tertiary formation0, seems to form its base, or at all events, is
lower than the present level of the sea, in the whole circumference
of the island. If we assume, that there are transition, or secondary
rocks beneath this tufa, and hidden by the sea, we must also
assume, in order to admit the pre-existence of Porto Santo as a
secondary formation, like Madeira, that the sea has been at a lower
level than it is now, or, that the shock which rent the previous
formation to admit of the throwing up of the clayey tufa, also
undermined these older rocks, and buried every vestige of the
former island beneath the ocean. When we associate the circumstance
of the shelly limestone, in the adjacent island of Baxo, being
160 feet above the sea, and about sixty feet above a similar deposit
of tufa, it seems to be much more natural to conclude, that both
Porto Santo and Baxo were formed beneath the ocean, and afterwards'hove
up, at a comparatively late period. Viewing Porto
Santo apart, it would be more simple to conclude, that the tufa,
(which is deposited confusedly, and not in beds) created by a submarine
volcano, was added, heap upon heap, and thus became
raised above the water ; in which case, (recollecting that it is found
at a height of 1600 feet above the sea, in the north-eastern parts
of the island) we must have admitted, that it continued to flow
through some crater or opening, long after the first emerging of
the island. But this reasoning is not applicable to the adjacent
island, which presents the shelly limestone above the tufa, and
which has evidently been separated from Porto Santo. The basalt
which caps the peaks, and descends in dikes, not only through the
0 Combined with the limestone and sandstone, it seems to resemble the local
marney formation described by D’Aubuisson, at the foot of the Pyrenees, more than
any other, (Traité de Gêognosie, vol. IL p. 436) ; unless it be the argile muriatifère,
sandstone, and shelly limestone breccias at Araya, near Cumana.—Humboldt, Relation
Historique, 1. 2, c. v.
N