Between Coquimbo and Guasco there are two considerable formations of mica-slate, in one of which the rock passed sometimes into common clay-slate and sometimes into a glossy black variety, very like that in the Chonos Archipelago.The folia and cleavage of these rocks ranged between [N.and N.W.by N.] and [S.and S.W.by S.].Near the Port of Guasco several varieties of altered clay-slate have a quite irregular cleavage.Between Guasco and Copiapo, there are some siliceous and talcaceous slates cleaving in a north and south line, with an easterly dip of between 60 and 70degrees: high up, also, the main valley of Copiapo, there is mica-slate with a high easterly dip.In the whole space between Valparaiso and Copiapo an easterly dip is much more common than an opposite or westerly one.
CONCLUDING REMARKS ON CLEAVAGE AND FOLIATION.
In this southern part of the southern hemisphere, we have seen that the cleavage-laminae range over wide areas with remarkable uniformity, cutting straight through the planes of stratification, but yet being parallel in strike to the main axes of elevation, and generally to the outlines of the coast.(In my paper on the Falkland Islands "Geological Journal" volume 3page 267, I have given a curious case on the authority of Captain Sulivan, R.N., of much folded beds of clay-slate, in some of which the cleavage is perpendicular to the horizon, and in others it is perpendicular to each curvature or fold of the bed: this appears a new case.) The dip, however, is as variable, both in angle and in direction (that is, sometimes being inclined to the one side and sometimes to the directly opposite side), as the strike is uniform.In all these respects there is a close agreement with the facts given by Professor Sedgwick in his celebrated memoir in the "Geological Transactions," and by Sir R.I.Murchison in his various excellent discussions on this subject.The Falkland Islands, and more especially Tierra del Fuego, offer striking instances of the lines of cleavage, the principle axes of elevation, and the outlines of the coast, gradually changing together their courses.The direction which prevails throughout Tierra del Fuego and the Falkland Islands, namely, from west with some northing to east with some southing, is also common to the several ridges in Northern Patagonia and in the western parts of Banda Oriental: in this latter province, in the Sierra Tapalguen, and in the Western Falkland Island, the W.by N., or W.N.W.and E.S.E., ridges, are crossed at right angles by others ranging N.N.E.and S.S.W.
The fact of the cleavage-laminae in the clay-slate of Tierra del Fuego, where seen cutting straight through the planes of stratification, and where consequently there could be no doubt about their nature, differing slightly in colour, texture, and hardness, appears to me very interesting.In a thick mass of laminated, feldspathic and altered clay-slate, interposed between two great strata of porphyritic conglomerate in Central Chile, and where there could be but little doubt about the bedding, I observed similar slight differences in composition, and likewise some distinct thin layers of epidote, parallel to the highly inclined cleavage of the mass.Again, Iincidentally noticed in North Wales, where glaciers had passed over the truncated edges of the highly inclined laminae of clay-slate, that the surface, though smooth, was worn into small parallel undulations, caused by the competent laminae being of slightly different degrees of hardness.
("London Philosophical Magazine" volume 21 page 182.) With reference to the slates of North Wales, Professor Sedgwick describes the planes of cleavage, as "coated over with chlorite and semi-crystalline matter, which not only merely define the planes in question, but strike in parallel flakes through the whole mass of the rock." ("Geological Transactions" volume 3 page 471.)In some of those glossy and hard varieties of clay-slate, which may often be seen passing into mica-schist, it has appeared to me that the cleavage-planes were formed of excessively thin, generally slighted convoluted, folia, composed of microscopically minute scales of mica.From these several facts, and more especially from the case of the clay-slate in Tierra del Fuego, it must, I think, be concluded, that the same power which has impressed on the slate its fissile structure or cleavage has tended to modify its mineralogical character in parallel planes.
Let us now turn to the foliation of the metamorphic schists, a subject which has been much less attended to.As in the case of cleavage-laminae, the folia preserve over very large areas a uniform strike: thus Humboldt found for a distance of 300 miles in Venezuela, and indeed over a much larger space, gneiss, granite, mica, and clay-slate, striking very uniformly N.E.and S.W., and dipping at an angle of between 60 and 70degrees to N.W.("Personal Narrative" volume 6 page 59 et seq.); it would even appear from the facts given in this chapter, that the metamorphic rocks throughout the north-eastern part of South America are generally foliated within two points of N.E.and S.W.Over the eastern parts of Banda Oriental, the foliation strikes with a high inclination, very uniformly N.N.E.to S.S.W., and over the western parts, in a W.by N.and E.by S.
