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To-day we travelled over porphyries like those of the last stage: but, about four miles from the last camp, steep sandstone rocks with excavations appeared on our left, at some distance from the river, from which they were separated by porphyry; but, farther on, they approached the river on both sides, and formed steep slopes, which compelled us to travel along the bed of the river itself.
The various Porphyries, and Diorites, and Granitic, and Sienitic rocks, which characterize large districts along the eastern coast of Australia, were missing; not a pebble, except of sandstone, was found in the numerous creeks and watercourses. Pieces of silicified wood were frequent in the bed of the Isaacs.
The threat was enough to raise a howl from the Egyptian; but this Porphyries soon put a stop to, and Sachepris, with perfect veracity, told her tale of all that had happened till Herse's return to the vessel.
At this period, the submarine craters yielded at intervals a prodigious supply of gypsum and other mineral exhalations, and occasionally, in certain places poured forth lavas, chiefly of a feldspathic nature: at this period, islands clothed with fir-trees and composed of porphyries, primary rocks, and the lower gypseous strata had already been locally upheaved, and exposed to the action of the waves; the general movement, however, at this time having been over a very wide area, one of slow subsidence, prolonged till the bed of the sea sank several thousand feet.
Well, the Youth sits down to his plans, and at once orders begin to fly across ocean to this port and that for the rarest marbles rosso antico from Mount Taenarus, verde antico from Thessally; with green Carystian, likewise shipped from Corinth; Carrara, Veronese Orange, Spanish broccatello, Derbyshire alabaster, black granite from Vyell's Cornish estate, red and purple porphyries from high up the Nile. . . . The Youth conjures up his gardens as by magic.
What are the affinities which unite the productions of modern volcanoes with the basalts, the phonolites, and those porphyries with bases of feldspar, which are without quartz, and which cover the Cordilleras of Peru and Mexico, as well as the small groups of the Monts Dores, of Cantal, and of Mezen in France?
Besides the true porphyries, there are many beds of earthy, quite white or yellowish, friable, easily fusible matter, resembling chalk, which under the microscope is seen to consist of minute broken crystals, and which, as remarked in a former chapter, singularly resembles the upper tufaceous beds of the Patagonian tertiary formation.
I may also give for example the African Brechia, which is a pudding-stone of the same nature. This stone is composed of granites or porphyries, serpentines and schisti, extremely indurated and perfectly consolidated.
I have repeatedly observed mountains of porphyries, more or less distinctly stratified towards their summits or on their flanks, without a trace of stratification in their central and basal parts: in most cases, I believe this is simply due either to the obliterating effects of metamorphic action, or to such parts having been mainly formed of intrusive porphyries, or to both causes conjoined; in some instances, however, it appeared to me very probable that the great central unstratified masses of porphyry were the now partially denuded nuclei of the old submarine volcanoes, and that the stratified parts marked the points whence the streams flowed.
The pebbles in the above conglomerates, both in the upper and lower beds, are all well rounded, and, though chiefly composed of various porphyries, there are some of red sandstone and of a jaspery stone, both like the rocks intercalated in layers in this same gypseous formation; there was one pebble of mica-slate and some of quartz, together with many particles of quartz.
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