Coral Reefs; Volcanic Islands; South American Geology — CompleteDarwin, Charles
Science
Coral Reefs; Volcanic Islands; South American Geology — Complete
Darwin, Charles
Beagle Expedition (1831-1836); Coral reefs and islands; Geology -- South America; Islands; Volcanoes
the crater down which it flowed, and probably even to a greater height.
It is interesting thus to trace the steps by which the structure of a
volcanic district becomes obscured, and finally obliterated: so near to
this last stage is St. Helena, that I believe no one has hitherto
suspected that the central ridge or axis of the island is the last
wreck of the crater, whence the most modern volcanic streams were
poured forth.
The great hollow space or valley southward of the central curved ridge,
across which the half of the crater must once have extended, is formed
of bare, water-worn hillocks and ridges of red, yellow, and brown
rocks, mingled together in chaos-like confusion, interlaced by dikes,
and without any regular stratification. The chief part consists of red
decomposing scoriæ, associated with various kinds of tuff and yellow
argillaceous beds, full of broken crystals, those of augite being
particularly large. Here and there masses of highly cellular and
amygdaloidal lavas protrude. From one of the ridges in the midst of the
valley, a conical precipitous hill, called Lot, boldly stands up, and
forms a most singular and conspicuous object. It is composed of
phonolite, divided in one part into great curved laminæ, in another,
into angular concretionary balls, and in a third part into outwardly
radiating columns. At its base the strata of lava, tuff, and scoriæ,
dip away on all sides;[9] the uncovered portion is 197 feet[10] in
height, and its horizontal section gives an oval figure. The phonolite
is of a greenish-grey
colour, and is full of minute acicular crystals of feldspar; in most
parts it has a conchoidal fracture, and is sonorous, yet it is
crenulated with minute air-cavities. In a S.W. direction from Lot,
there are some other remarkable columnar pinnacles, but of a less
regular shape, namely, Lot’s Wife, and the Asses’ Ears, composed of
allied kinds of rock. From their flattened shape, and their relative
position to each other, they are evidently connected on the same line
of fissure. It is, moreover, remarkable that this same N.E. and S.W.
line, joining Lot and Lot’s Wife, if prolonged would intersect
Flagstaff Hill, which, as before stated, is crossed by numerous dikes
running in this direction, and which has a disturbed structure,
rendering it probable that a great body of once fluid rock lies
injected beneath it.
[9] Abich in his “Views of Vesuvius” (plate vi), has shown the manner
in which beds, under nearly similar circumstances, are tilted up. The
upper beds are more turned up than the lower; and he accounts for
this, by showing that the lava insinuates itself horizontally between
the lower beds.
[10] This height is given by Mr. Seale in his Geognosy of the island.
The height of the summit above the level of the sea is said to be
1,444 feet.
Public-domain text, read in full here on John Shaqi.
Reviews
Reviews
No reviews yet
Be the first to share your thoughts on this work.
Elsewhere in the archive
Join the Discussion
Join the discussion
Sign in to leave a comment or review.
Sign InorCreate an account