第96章 LUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIA

M.d'Orbigny states that MM.Hombron and Grange found in this neighbourhood an Ancyloceras, perhaps A.simplex, an Ammonite, a Plicatula and Modiola.

("Voyage" Part Geolog.page 242.) M.d'Orbigny believes from the general character of these fossils, and from the Ancyloceras being identical (as far as its imperfect condition allows of comparison) with the A.simplex of Europe, that the formation belongs to an early stage of the Cretaceous system.Professor E.Forbes, judging only from my specimens, concurs in the probability of this conclusion.The Hamites elatior of the above list, of which a description has been given by Mr.Sowerby, and which is remarkable from its large size, has not been seen either by M.d'Orbigny or Professor E.Forbes, as, since my return to England, the specimens have been lost.

The great clay-slate formation of Tierra del Fuego being cretaceous, is certainly a very interesting fact,--whether we consider the appearance of the country, which, without the evidence afforded by the fossils, would form the analogy of most known districts, probably have been considered as belonging to the Palaeozoic series,--or whether we view it as showing that the age of this terminal portion of the great axis of South America, is the same (as will hereafter be seen) with the Cordillera of Chile and Peru.

The clay-slate in many parts of Tierra del Fuego, is broken by dikes and by great masses of greenstone, often highly hornblendic (In a greenstone-dike in the Magdalen Channel, the feldspar cleaved with the angle of albite.

This dike was crossed, as well as the surrounding slate, by a large vein of quartz, a circumstance of unusual occurrence.): almost all the small islets within the clay-slate districts are thus composed.The slate near the dikes generally becomes paler-coloured, harder, less fissile, of a feldspathic nature, and passes into a porphyry or greenstone: in one case, however, it became more fissile, of a red colour, and contained minute scales of mica, which were absent in the unaltered rock.On the east side of Ponsonby Sound some dikes composed of a pale sonorous feldspathic rock, porphyritic with a little feldspar, were remarkable from their number,--there being within the space of a mile at least one hundred,--from their nearly equalling in bulk the intermediate slate,--and more especially from the excessive fineness (like the finest inlaid carpentry) and perfect parallelism of their junctions with the almost vertical laminae of clay-slate.I was unable to persuade myself that these great parallel masses had been injected, until Ifound one dike which abruptly thinned out to half its thickness, and had one of its walls jagged, with fragments of the slate embedded in it.

In Southern Tierra del Fuego, the clay-slate towards its S.W.boundary, becomes much altered and feldspathic.Thus on Wollaston Island slate and grauwacke can be distinctly traced passing into feldspathic rocks and greenstones, including iron pyrites and epidote, but still retaining traces of cleavage with the usual strike and dip.One such metamorphosed mass was traversed by large vein-like masses of a beautiful mixture (as ascertained by Professor Miller) of green epidote, garnets, and white calcareous spar.

On the northern point of this same island, there were various ancient submarine volcanic rocks, consisting of amygdaloids with dark bole and agate,--of basalt with decomposed olivine--of compact lava with glassy feldspar,--and of a coarse conglomerate of red scoriae, parts being amygdaloidal with carbonate of lime.The southern part of Wollaston Island and the whole of Hermite and Horn Islands, seem formed of cones of greenstone; the outlying islets of Il Defenso and D.Raminez are said to consist of porphyritic lava.(Determined by Professor Jameson.Weddell's "Voyage" page 169.) In crossing Hardy Peninsula, the slate still retaining traces of its usual cleavage, passes into columnar feldspathic rocks, which are succeeded by an irregular tract of trappean and basaltic rocks, containing glassy feldspar and much iron pyrites: there is, also, some harsh red claystone porphyry, and an almost true trachyte, with needles of hornblende, and in one spot a curious slaty rock divided into quadrangular columns, having a base almost like trachyte, with drusy cavities lined by crystals, too imperfect, according to Professor Miller, to be measured, but resembling Zeagonite.(See Mr.Brooke's Paper in the "London Philosophical Magazine" volume 10.This mineral occurs in an ancient volcanic rock near Rome.) In the midst of these singular rocks, no doubt of ancient submarine volcanic origin, a high hill of feldspathic clay-slate projected, retaining its usual cleavage.Near this point, there was a small hillock, having the aspect of granite, but formed of white albite, brilliant crystals of hornblende (both ascertained by the reflecting goniometer) and mica; but with no quartz.No recent volcanic district has been observed in any part of Tierra del Fuego.

Five miles west of the bifurcation of the Beagle Channel, the slate-formation, instead of becoming, as in the more southern parts of Tierra del Fuego, feldspathic, and associated with trappean or old volcanic rocks, passes by alternations into a great underlying mass of fine gneiss and glossy clay-slate, which at no great distance is succeeded by a grand formation of mica-slate containing garnets.The folia of these metamorphic schists strike parallel to the cleavage-planes of the clay-slate, which have a very uniform direction over the whole of this part of the country:

the folia, however, are undulatory and tortuous, whilst the cleavage-laminae of the slate are straight.These schists compose the chief mountain-chain of Southern Tierra del Fuego, ranging along the north side of the northern arm of the Beagle Channel, in a short W.N.W.and E.S.E.