第98章 LUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIA

The clay-slate cleaves in the same W.N.W.and E.S.E.direction, as on Navarin Island, on both sides of the Beagle Channel, on the eastern side of Hoste Island, on the N.E.side of Hardy Peninsula, and on the northern point of Wollaston Island; although in these two latter localities the cleavage has been much obscured by the metamorphosed and feldspathic condition of the slate.Within the area of these several islands, including Navarin Island, the direction of the stratification and of the mountain-chains is very obscure; though the mountains in several places appeared to range in the same W.N.W.line with the cleavage: the outline of the coast, however, does not correspond with this line.Near the bifurcation of the Beagle Channel, where the underlying metamorphic schists are first seen, they are foliated (with some irregularities), in this same W.N.W.line, and parallel, as before stated, to the main mountain-axis of this part of the country.Westward of this main range, the metamorphic schists are foliated, though less plainly, in the same direction, which is likewise common to the zone of old erupted trappean rocks, forming the outermost islets.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.

Further northward, near Port Famine, the stratification of the clay-slate and of the associated rocks, is well defined, and there alone the cleavage and strata-planes are parallel.A little north of this port there is an anticlinal axis ranging N.W.(or a little more westerly) and S.E.: south of the port, as far as Admiralty Sound and Gabriel Channel, the outline of the land clearly indicates the existence of several lines of elevation in this same N.W.direction, which, I may add, is so uniform in the western half of the St.of Magellan, that, as Captain King has remarked, "a parallel ruler placed on the map upon the projecting points of the south shore, and extended across the strait, will also touch the headlands on the opposite coast." ("Geographical Journal" volume 1 page 170.) It would appear, from Captain King's observations, that over all this area the cleavage extends in the same line.Deep-water channels, however, in all parts of Tierra del Fuego have burst through the trammels both of stratification and cleavage;most of them may have been formed during the elevation of the land by long-continued erosion, but others, for instance the Beagle Channel, which stretches like a narrow canal for 120 miles obliquely through the mountains, can hardly have thus originated.

Finally, we have seen that in the extreme eastern point of Tierra del Fuego, the cleavage and coast-lines extend W.and E.and even W.S.W.and E.N.E.: over a large area westward, the cleavage, the main range of mountains, and some subordinate ranges, but not the outlines of the coast, strike W.N.W., and E.S.E.: in the central and western parts of the St.of Magellan, the stratification, the mountain-ranges, the outlines of the coast, and the cleavage all strike nearly N.W.and S.E.North of the strait, the outline of the coast, and the mountains on the mainland, run nearly north and south.Hence we see, at this southern point of the continent, how gradually the Cordillera bend, from their north and south course of so many thousand miles in length, into an E.and even E.N.E.

direction.

WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE.

The first place at which we landed north of the St.of Magellan was near Cape Tres Montes, in latitude 47 degrees S.Between this point and the Northern Chonos Islands, a distance of 200 miles, the "Beagle" visited several points, and specimens were collected for me from the intermediate spaces by Lieutenant Stokes.The predominant rock is mica-slate, with thick folia of quartz, very frequently alternating with and passing into a chloritic, or into a black, glossy, often striated, slightly anthracitic schist, which soils paper, and becomes white under a great heat, and then fuses.Thin layers of feldspar, swelling at intervals into well crystallised kernels, are sometimes included in these black schists; and Iobserved one mass of the ordinary black variety insensibly lose its fissile structure, and pass into a singular mixture of chlorite, epidote, feldspar, and mica.Great veins of quartz are numerous in the mica-schists; wherever these occur the folia are much convoluted.In the southern part of the Peninsula of Tres Montes, a compact altered feldspathic rock with crystals of feldspar and grains of quartz is the commonest variety; this rock exhibits occasionally traces of an original brecciated structure, and often presents (like the altered state of Tierra del Fuego) traces of cleavage-planes, which strike in the same direction with the folia of mica-schist further northward.(The peculiar, abruptly conical form of the hills in this neighbourhood, would have led any one at first to have supposed that they had been formed of injected or intrusive rocks.At Inchemo Island, a similar rock gradually becomes granulo-crystalline and acquires scales of mica; and this variety at S.Estevan becomes highly laminated, and though still exhibiting some rounded grains of quartz, passes into the black, glossy, slightly anthracitic schist, which, as we have seen, repeatedly alternates with and passes into the micaceous and chloritic schists.Hence all the rocks on this line of coast belong to one series, and insensibly vary from an altered feldspathic clay-slate into largely foliated, true mica-schist.