第62章

TRACHYTE AND BASALT.--DISTRIBUTION OF VOLCANIC ISLES.

The sinking of crystals in fluid lava.

Specific gravity of the constituent parts of trachyte and of basalt, and their consequent separation.

Obsidian.

Apparent non-separation of the elements of plutonic rocks.

Origin of trap-dikes in the plutonic series.

Distribution of volcanic islands; their prevalence in the great oceans.

They are generally arranged in lines.

The central volcanoes of Von Buch doubtful.

Volcanic islands bordering continents.

Antiquity of volcanic islands, and their elevation in mass.

Eruptions on parallel lines of fissure within the same geological period.

ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO THEIRSPECIFIC GRAVITIES.

One side of Fresh-water Bay, in James Island, is formed by the wreck of a large crater, mentioned in the last chapter, of which the interior has been filled up by a pool of basalt, about two hundred feet in thickness.This basalt is of a grey colour, and contains many crystals of glassy albite, which become much more numerous in the lower, scoriaceous part.This is contrary to what might have been expected, for if the crystals had been originally disseminated in equal numbers, the greater intumescence of this lower scoriaceous part would have made them appear fewer in number.Von Buch has described a stream of obsidian on the Peak of Teneriffe, in which the crystals of feldspar become more and more numerous, as the depth or thickness increases, so that near the lower surface of the stream the lava even resembles a primary rock.("Description des Isles Canaries" pages 190and 191.) Von Buch further states, that M.Dree, in his experiments in melting lava, found that the crystals of feldspar always tended to precipitate themselves to the bottom of the crucible.In these cases, Ipresume there can be no doubt that the crystals sink from their weight.(In a mass of molten iron, it is found ("Edinburgh New Philosophical Journal"volume 24 page 66) that the substances, which have a closer affinity for oxygen than iron has, rise from the interior of the mass to the surface.

But a similar cause can hardly apply to the separation of the crystals of these lava-streams.The cooling of the surface of lava seems, in some cases, to have affected its composition; for Dufrenoy ("Mem.pour servir"tome 4 page 271) found that the interior parts of a stream near Naples contained two-thirds of a mineral which was acted on by acids, whilst the surface consisted chiefly of a mineral unattackable by acids.) The specific gravity of feldspar varies from 2.4 to 2.58, whilst obsidian seems commonly to be from 2.3 to 2.4; and in a fluidified state its specific gravity would probably be less, which would facilitate the sinking of the crystals of feldspar.(I have taken the specific gravities of the simple minerals from Von Kobell, one of the latest and best authorities, and of the rocks from various authorities.Obsidian, according to Phillips, is 2.35; and Jameson says it never exceeds 2.4; but a specimen from Ascension, weighed by myself, was 2.42.) At James Island, the crystals of albite, though no doubt of less weight than the grey basalt, in the parts where compact, might easily be of greater specific gravity than the scoriaceous mass, formed of melted lava and bubbles of heated gas.