第30章

The soft white stone above mentioned is remarkable from its singular resemblance, when viewed in mass, to a sedimentary tuff: it was long before I could persuade myself that such was not its origin; and other geologists have been perplexed by closely similar formations in trachytic regions.In two cases, this white earthy stone formed isolated hills; in a third, it was associated with columnar and laminated trachyte; but I was unable to trace an actual junction.It contains numerous crystals of glassy feldspar and black microscopical specks, and is marked with small darker patches, exactly as in the surrounding trachyte.Its basis, however, when viewed under the microscope, is generally quite earthy; but sometimes it exhibits a decidedly crystalline structure.On the hill marked "Crater of an old volcano," it passes into a pale greenish-grey variety, differing only in its colour, and in not being so earthy; the passage was in one case effected insensibly; in another, it was formed by numerous, rounded and angular, masses of the greenish variety, being embedded in the white variety;--in this latter case, the appearance was very much like that of a sedimentary deposit, torn up and abraded during the deposition of a subsequent stratum.Both these varieties are traversed by innumerable tortuous veins (presently to be described), which are totally unlike injected dikes, or indeed any other veins which I have ever seen.Both varieties include a few scattered fragments, large and small, of dark-coloured scoriaceous rocks, the cells of some of which are partially filled with the white earthy stone; they likewise include some huge blocks of a cellular porphyry.(The porphyry is dark coloured; it contains numerous, often fractured, crystals of white opaque feldspar, also decomposing crystals of oxide of iron; its vesicles include masses of delicate, hair-like, crystals, apparently of analcime.) These fragments project from the weathered surface, and perfectly resemble fragments embedded in a true sedimentary tuff.But as it is known that extraneous fragments of cellular rock are sometimes included in columnar trachyte, in phonolite (D'Aubuisson "Traite de Geognosie" tome 2 page 548.), and in other compact lavas, this circumstance is not any real argument for the sedimentary origin of the white earthy stone.(Dr.Daubeny on Volcanoes, page 180 seems to have been led to believe that certain trachytic formations of Ischia and of the Puy de Dome, which closely resemble these of Ascension, were of sedimentary origin, chiefly from the frequent presence in them "of scoriform portions, different in colour from the matrix." Dr.Daubeny adds, that on the other hand, Brocchi, and other eminent geologists, have considered these beds as earthy varieties of trachyte; he considers the subject deserving of further attention.) The insensible passage of the greenish variety into the white one, and likewise the more abrupt passage by fragments of the former being embedded in the latter, might result from slight differences in the composition of the same mass of molten stone, and from the abrading action of one such part still fluid on another part already solidified.The curiously formed veins have, I believe, been formed by siliceous matter being subsequently segregated.But my chief reason for believing that these soft earthy stones, with their extraneous fragments, are not of sedimentary origin, is the extreme improbability of crystals of feldspar, black microscopical specks, and small stains of a darker colour occurring in the same proportional numbers in an aqueous deposit, and in masses of solid trachyte.Moreover, as I have remarked, the microscope occasionally reveals a crystalline structure in the apparently earthy basis.On the other hand, the partial decomposition of such great masses of trachyte, forming whole mountains, is undoubtedly a circumstance of not easy explanation.

VEINS IN THE EARTHY TRACHYTIC MASSES.