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Updated: July 17, 2025
The numerous islands of this group, with the exception of Lemus, Ypun, consist of metamorphic schists; these two islands are formed of softish grey and brown, fusible, often laminated sandstones, containing a few pebbles, fragments of black lignite, and numerous mammillated concretions of hard calcareous sandstone. Voluta, fragments of.
The river-terrace is in all respects similar to that at the junction of the Tambur and Khawa, but very extensive: the stones it contained were of all sizes, from a nut to huge boulders upwards of fifteen feet long, of which many strewed the surface, while others were in the bed of the river: all were of gneiss, quartz, and granite, and had doubtless been transported from great elevations, as the rocks in situ both here and for several thousand feet higher up the river were micaceous schists, dipping in various directions, and at all angles, with, however, a general strike to the north-west.
They are the same in material, but only changed into new forms and combinations; hence they have been called by Mr. Lyell, metamorphic rocks. But how comes it that some of them are composed almost exclusively of one of the materials of granite; the mica schists, for example, of mica the quartz rocks, of quartz, &c.? For this there are both chemical and mechanical causes.
But still we marched on, and I alone was forgetting the length of the way by losing myself in the midst of geological contemplations. The temperature remained what it had been during our passage through the lava and schists. Only my sense of smell was forcibly affected by an odour of protocarburet of hydrogen.
Great unconformities, UU' separate the Algonkian both from the Archean and from the Cambrian, and divide it into three distinct systems, the LOWER HURONIAN, the UPPER HURONIAN, and the KEWEENAWAN. The Lower and the Upper Huronian consist in the main of old sea muds and sands and limy oozes now changed to gneisses, schists, marbles, quartzites, slates, and other metamorphic rocks.
Hence the area, over which the cleavage of the slate and the foliation of the metamorphic schists extends with an average W.N.W. and E.S.E. strike, is about forty miles in a north and south line, and ninety miles in an east and west line.
On the south-western side of this northern arm of the Beagle Channel, the clay-slate is seen with its STRATA dipping from the great chain, so that the metamorphic schists here form a ridge bordered on each side by clay-slate.
It consists of foliated rocks, gneisses, and schists of various kinds, which have been cut with dikes and other intrusions of molten rock, and have been broken, crumpled, and crushed, and left in interlocking masses so confused that their true arrangement can usually be made out only with the greatest difficulty if at all.
The trachyte of the Drachenfels was probably the neck of a volcano which burst through the fundamental schists of the Devonian period.
The fundamental rocks of this region belong to the Silurian, Devonian, and Carboniferous systems, consisting of schists, grits, and limestones, with occasional horizontal beds of Miocene sandstone and shale with lignite, resting on the upturned edges of the older rocks.
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