第90章 LUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIA

Schomburgk's observations see "Geographical Journal" 1842 page 190.See also Humboldt's discussion on Loxodrism in the "Personal Narrative.") Hence all the observations hitherto made tend to show that the gneissic rocks over the whole of this part of the continent have their folia extending generally within almost a point of the compass of the same direction.(Ilanded at only one place north of Bahia, namely, at Pernambuco.I found there only soft, horizontally stratified matter, formed from disintegrated granitic rocks, and some yellowish impure limestone, probably of a tertiary epoch.I have described a most singular natural bar of hard sandstone, which protects the harbour, in the 19th volume 1841 page 258 of the "London and Edinburgh Philosophical Magazine."ABROLHOS ISLETS, Latitude 18 degrees S.off the coast of Brazil.

Although not strictly in place, I do not know where I can more conveniently describe this little group of small islands.The lowest bed is a sandstone with ferruginous veins; it weathers into an extraordinary honeycombed mass;above it there is a dark-coloured argillaceous shale; above this a coarser sandstone--making a total thickness of about sixty feet; and lastly, above these sedimentary beds, there is a fine conformable mass of greenstone, in some parts having a columnar structure.All the strata, as well as the surface of the land, dip at an angle of about 12 degrees to N.by W.Some of the islets are composed entirely of the sedimentary, others of the trappean rocks, generally, however, with the sandstone, cropping out on the southern shores.)RIO DE JANEIRO.

This whole district is almost exclusively formed of gneiss, abounding with garnets, and porphyritic with large crystals, even three and four inches in length, of orthoclase feldspar: in these crystals mica and garnets are often enclosed.At the western base of the Corcovado, there is some ferruginous carious quartz-rock; and in the Tijeuka range, much fine-grained granite.I observed boulders of greenstone in several places; and on the islet of Villegagnon, and likewise on the coast some miles northward, two large trappean dikes.The porphyritic gneiss, or gneiss-granite as it has been called by Humboldt, is only so far foliated that the constituent minerals are arranged with a certain degree of regularity, and may be said to have a "GRAIN," but they are not separated into distinct folia or laminae.There are, however, several other varieties of gneiss regularly foliated, and alternating with each other in so-called strata.

The stratification and foliation of the ordinary gneisses, and the foliation or "grain" of the gneiss-granite, are parallel to each other, and generally strike within a point of N.E.and S.W.dipping at a high angle (between 50 and 60 degrees) generally to S.E.: so that here again we meet with the strike so prevalent over the more northern parts of this continent.The mountains of gneiss-granite are to a remarkable degree abruptly conical, which seems caused by the rock tending to exfoliate in thick, conically concentric layers: these peaks resemble in shape those of phonolite and other injected rocks on volcanic islands; nor is the grain or foliation (as we shall afterwards see) any difficulty on the idea of the gneiss-granite having been an intrusive rather than a metamorphic formation.The lines of mountains, but not always each separate hill, range nearly in the same direction with the foliation and so-called stratification, but rather more easterly.

(FIGURE 22.FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE SAMEROCK.)

On a bare gently inclined surface of the porphyritic gneiss in Botofogo Bay, I observed the appearance represented in Figure 22.

A fragment seven yards long and two in width, with angular and distinctly defined edges, composed of a peculiar variety of gneiss with dark layers of mica and garnets, is surrounded on all sides by the ordinary gneiss-granite; both having been dislocated by a granitic vein.The folia in the fragment and in the surrounding rock strike in the same N.N.E.and S.S.W.

line; but in the fragment they are vertical, whereas in the gneiss-granite they dip at a small angle, as shown by the arrows, to S.S.E.This fragment, considering its great size, its solitary position, and its foliated structure parallel to that of the surrounding rock, is, as far as I know, a unique case: and I will not attempt any explanation of its origin.

The numerous travellers in this country, have all been greatly surprised at the depth to which the gneiss and other granitic rocks, as well as the talcose slates of the interior, have been decomposed.(Spix and Martius have collected in an Appendix to their "Travels," the largest body of facts on this subject.See also some remarks by M.Lund in his communications to the Academy at Copenhagen; and others by M.Gaudichaud in Freycinet "Voyage.") Near Rio, every mineral except the quartz has been completely softened, in some places to a depth little less than one hundred feet.(Dr.