第95章 LUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIA

Some miles up the harbour, in a line of cliffs formed of slightly metamorphosed tufaceous and porphyritic claystone beds, I observed three vertical dikes, so closely resembling in general appearance ordinary volcanic dikes, that I did not doubt, until closely examining their composition, that they had been injected from below.The first is straight, with parallel sides, and about four feet wide; it consists of whitish, indurated tufaceous matter, precisely like some of the beds intersected by it.The second dike is more remarkable; it is slightly tortuous, about eighteen inches thick, and can be traced for a considerable distance along the beach; it is of a purplish-red or brown colour, and is formed chiefly of ROUNDED grains of quartz, with broken crystals of earthy feldspar, scales of black mica, and minute fragments of claystone porphyry, all firmly united together in a hard sparing base.The structure of this dike shows obviously that it is of mechanical and sedimentary origin; yet it thinned out upwards, and did not cut through the uppermost strata in the cliffs.This fact at first appears to indicate that the matter could not have been washed in from above (Upfilled fissures are known to occur both in volcanic and in ordinary sedimentary formations.At the Galapagos Archipelago "Volcanic Islands" etc., there are some striking examples of pseudo-dikes composed of hard tuff.); but if we reflect on the suction which would result from a deep-seated fissure being formed, we may admit that if the fissure were in any part open to the surface, mud and water might well be drawn into it along its whole course.The third dike consisted of a hard, rough, white rock, almost composed of broken crystals of glassy feldspar, with numerous scales of black mica, cemented in a scanty base; there was little in the appearance of this rock, to preclude the idea of its having been a true injected feldspathic dike.The matter composing these three pseudo-dikes, especially the second one, appears to have suffered, like the surrounding strata, a certain degree of metamorphic action; and this has much aided the deceptive appearance.At Bahia, in Brazil, we have seen that a true injected hornblendic dike, not only has suffered metamorphosis, but has been dislocated and even diffused in the surrounding gneiss, under the form of separate crystals and of fragments.

FALKLAND ISLANDS.

I have described these islands in a paper published in the third volume of the "Geological Journal." The mountain-ridges consist of quartz, and the lower country of clay-slate and sandstone, the latter containing Palaeozoic fossils.These fossils have been separately described by Messrs.Morris and Sharpe: some of them resemble Silurian, and others Devonian forms.In the eastern part of the group the several parallel ridges of quartz extend in a west and east line; but further westward the line becomes W.N.W.and E.S.E., and even still more northerly.The cleavage-planes of the clay-slate are highly inclined, generally at an angle of above 50 degrees, and often vertical; they strike almost invariably in the same direction with the quartz ranges.The outline of the indented shores of the two main islands, and the relative positions of the smaller islets, accord with the strike both of the main axes of elevation and of the cleavage of the clay-slate.

TIERRA DEL FUEGO.

My notes on the geology of this country are copious, but as they are unimportant, and as fossils were found only in one district, a brief sketch will be here sufficient.The east coast from the S.of Magellan (where the boulder formation is largely developed) to St.Polycarp's Bay is formed of horizontal tertiary strata, bounded some way towards the interior by a broad mountainous band of clay-slate.This great clay-slate formation extends from St.Le Maire westward for 140 miles, along both sides of the Beagle Channel to near its bifurcation.South of this channel, it forms all Navarin Island, and the eastern half of Hoste Island and of Hardy Peninsula; north of the Beagle Channel it extends in a north-west line on both sides of Admiralty Sound to Brunswick Peninsula in the St.of Magellan, and I have reason to believe, stretches far up the eastern side of the Cordillera.The western and broken side of Tierra del Fuego towards the Pacific is formed of metamorphic schists, granite and various trappean rocks: the line of separation between the crystalline and clay-slate formations can generally be distinguished, as remarked by Captain King, by the parallelism in the clay-slate districts of the shores and channels, ranging in a line between [W.20 degrees to 40 degrees N.] and [E.20degrees to 40 degrees S.].("Geographical Journal" volume 1 page 155.)The clay-slate is generally fissile, sometimes siliceous or ferruginous, with veins of quartz and calcareous spar; it often assumes, especially on the loftier mountains, an altered feldspathic character, passing into feldspathic porphyry: occasionally it is associated with breccia and grauwacke.At Good Success Bay, there is a little intercalated black crystalline limestone.At Port Famine much of the clay-slate is calcareous, and passes either into a mudstone or into grauwacke, including odd-shaped concretions of dark argillaceous limestone.Here alone, on the shore a few miles north of Port Famine, and on the summit of Mount Tarn (2,600 feet high), I found organic remains; they consist of:--1.Ancyloceras simplex, d'Orbigny "Pal Franc" Mount Tarn.

2.Fusus (in imperfect state), d'Orbigny "Pal Franc" Mount Tarn.

3.Natica, d'Orbigny "Pal Franc" Mount Tarn.

4.Pentacrimus, d'Orbigny "Pal Franc" Mount Tarn.

5.Lucina excentrica, G.B.Sowerby, Port Famine.

6.Venus (in imperfect state), G.B.Sowerby, Port Famine.

7.Turbinolia (?), G.B.Sowerby, Port Famine.

8.Hamites elatior, G.B.Sowerby, Port Famine.