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The hill immediately over our camp was Bullabalakit, and consisted partly of granular felspar, probably tinged greenish with chlorite; and partly of concretionary porphyry, the concretions being mottled red and white, and containing grains of quartz and crystals of common felspar; the white concretions resisting the action of the atmosphere stood in relief on the weather surface; I noticed also a vein of amethystine quartz.
I am tempted briefly to describe three other singular allied varieties of rock; the first without examination would have passed for a stratified porphyritic breccia, but all the included angular fragments consisted of a border of pinkish crystalline feldspathic matter, surrounding a dark translucent siliceous centre, in which grains of quartz not quite blended into the paste could be distinguished: this uniformity in the nature of the fragments shows that they are not of mechanical, but of concretionary origin, having resulted perhaps from the self-breaking up and aggregation of layers of indurated tuff containing numerous grains of quartz, into which, indeed, the whole mass in one part passed.
The prevailing rock has a compact purplish base, with crystals of earthy or opaque feldspar, and often with grains of quartz. There are other varieties, with an almost truly trachytic base, full of little angular vesicles and crystals of glassy feldspar; and there are beds of black perfect pitchstone, as well as of a concretionary imperfect variety.
This was a very convenient station, being nearly on the margin of the river, the bank of which, consisting of concretionary limestone, afforded easy access for the cattle to the water while surrounding hollows supplied them with plenty of grass. I was now enabled to reduce the cattle guard from four to two men, which was a great relief to them.
But changes like these in the Lym-Fiord, and those before mentioned as occurring at the mouths of great rivers, will only account for some cases of marine deposits of partial extent resting on fresh-water strata. Chemical and Mechanical Deposits. Cementing together of Particles. Hardening by Exposure to Air. Concretionary Nodules. Consolidating Effects of Pressure.
If consolidation takes place in B, the concretionary action may spread upward into a part of A, where the carbonate of lime is more abundant than in the rest; so that a mass, d e f, forming a portion of the superior stratum, becomes united with B into one solid mass of stone.
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 laminae, in another, into angular concretionary balls, and in a third part into outwardly radiating columns. Seale in his Geognosy of the island.
At Rosario there is but little tosca-rock: near this place I first noticed at the edge of the river traces of an underlying formation, which, twenty- five miles higher up in the estancia of Gorodona, consists of a pale yellowish clay, abounding with concretionary cylinders of a ferruginous sandstone.
A red, quartzose, fine-grained conglomerate, with grains of quartz, and with patches of white earthy feldspar, apparently due to some process of concretionary crystalline action; this bed is more compact and metamorphosed than any of the overlying conglomerates. 5th. A whitish cherty limestone, with nodules of bluish argillaceous limestone. 6th.
Near Valparaiso the prevailing rock is gneiss, generally including much hornblende: concretionary balls formed of feldspar, hornblende and mica, from two or three feet in diameter, are in very many places conformably enfolded by the foliated gneiss: veins of quartz and feldspar, including black schorl and well-crystallised epidote, are numerous.
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