Beagle Expedition (1831-1836); Geology -- South America; Volcanoes
This lava is of a pale grey colour, fusing into a black enamel; its
fracture is rather earthy and concretionary; it contains olivine in
small grains. The central parts of the mass are compact, or at most
crenulated with a few minute cavities, and are often columnar. At Quail
Island this structure was assumed in a striking manner; the lava in one
part being divided into horizontal laminae, which became in another
part split by vertical fissures into five-sided plates; and these
again, being piled on each other, insensibly became soldered together,
forming fine symmetrical columns. The lower surface of the lava is
vesicular, but sometimes only to the thickness of a few inches; the
upper surface, which is likewise vesicular, is divided into balls,
frequently as much as three feet in diameter, made up of concentric
layers. The mass is composed of more than one stream; its total
thickness being, on an average, about eighty feet: the lower portion
has certainly flowed beneath the sea, and probably likewise the upper
portion. The chief part of this lava has flowed from the central
districts, between the hills marked A, B, C, etc., in the woodcut- map.
The surface of the country, near the coast, is level and barren;
towards the interior, the land rises by successive terraces, of which
four, when viewed from a distance, could be distinctly counted.
VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THE
EJECTED MATTER ASSOCIATED WITH EARTHY LIME.
These recent lavas have proceeded from those scattered, conical,
reddish- coloured hills, which rise abruptly from the plain-country
near the coast. I ascended some of them, but will describe only one,
namely, RED HILL, which may serve as a type of its class, and is
remarkable in some especial respects. Its height is about six hundred
feet; it is composed of bright red, highly scoriaceous rock of a
basaltic nature; on one side of its summit there is a hollow, probably
the last remnant of a crater. Several of the other hills of this class,
judging from their external forms, are surmounted by much more perfect
craters. When sailing along the coast, it was evident that a
considerable body of lava had flowed from Red Hill, over a line of
cliff about one hundred and twenty feet in height, into the sea: this
line of cliff is continuous with that forming the coast, and bounding
the plain on both sides of this hill; these streams, therefore, were
erupted, after the formation of the coast-cliffs, from Red Hill, when
it must have stood, as it now does, above the level of the sea. This
conclusion accords with the highly scoriaceous condition of all the
rock on it, appearing to be of subaerial formation: and this is
important, as there are some beds of calcareous matter near its summit,
which might, at a hasty glance, have been mistaken for a submarine
deposit. These beds consist of white, earthy, carbonate of lime,
extremely friable so as to be crushed with the least pressure; the most
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