第89章 LUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIA

Brazil, Bahia, gneiss with disjointed metamorphosed dikes.

Strike of foliation.

Rio de Janeiro, gneiss-granite, embedded fragment in, decomposition of.

La Plata, metamorphic and old volcanic rocks of.

S.Ventana.

Claystone porphyry formation of Patagonia; singular metamorphic rocks;pseudo-dikes.

Falkland Islands, Palaeozoic fossils of.

Tierra del Fuego, clay-slate formation, cretaceous fossils of; cleavage and foliation; form of land.

Chonos Archipelago, mica-schists, foliation disturbed by granitic axis;dikes.

Chiloe.

Concepcion, dikes, successive formation of.

Central and Northern Chile.

Concluding remarks on cleavage and foliation.

Their close analogy and similar origin.

Stratification of metamorphic schists.

Foliation of intrusive rocks.

Relation of cleavage and foliation to the lines of tension during metamorphosis.

The metamorphic and plutonic formations of the several districts visited by the "Beagle" will be here chiefly treated of, but only such cases as appear to me new, or of some special interest, will be described in detail; at the end of the chapter I will sum up all the facts on cleavage and foliation,--to which I particularly attended.

BAHIA, BRAZIL: latitude 13 degrees south.

The prevailing rock is gneiss, often passing, by the disappearance of the quartz and mica, and by the feldspar losing its red colour, into a brilliantly grey primitive greenstone.Not unfrequently quartz and hornblende are arranged in layers in almost amorphous feldspar.There is some fine-grained syenitic granite, orbicularly marked by ferruginous lines, and weathering into vertical, cylindrical holes, almost touching each other.In the gneiss, concretions of granular feldspar and others of garnets with mica occur.The gneiss is traversed by numerous dikes composed of black, finely crystallised, hornblendic rock, containing a little glassy feldspar and sometimes mica, and varying in thickness from mere threads to ten feet: these threads, which are often curvilinear, could sometimes be traced running into the larger dikes.One of these dikes was remarkable from having been in two or three places laterally disjointed, with unbroken gneiss interposed between the broken ends, and in one part with a portion of the gneiss driven, apparently whilst in a softened state, into its side or wall.In several neighbouring places, the gneiss included angular, well-defined, sometimes bent, masses of hornblende rock, quite like, except in being more perfectly crystallised, that forming the dikes, and, at least in one instance, containing (as determined by Professor Miller) augite as well as hornblende.In one or two cases these angular masses, though now quite separate from each other by the solid gneiss, had, from their exact correspondence in size and shape, evidently once been united; hence Icannot doubt that most or all of the fragments have been derived from the breaking up of the dikes, of which we see the first stage in the above-mentioned laterally disjointed one.The gneiss close to the fragments generally contained many large crystals of hornblende, which are entirely absent or rare in other parts: its folia or laminae were gently bent round the fragments, in the same manner as they sometimes are round concretions.

Hence the gneiss has certainly been softened, its composition modified, and its folia arranged, subsequently to the breaking up of the dikes, these latter also having been at the same time bent and softened.(Professor Hitchcock "Geology of Massachusetts" volume 2 page 673, gives a closely similar case of a greenstone dike in syenite.)I must here take the opportunity of premising, that by the term CLEAVAGE Iimply those planes of division which render a rock, appearing to the eye quite or nearly homogeneous, fissile.By the term FOLIATION, I refer to the layers or plates of different mineralogical nature of which most metamorphic schists are composed; there are, also, often included in such masses, alternating, homogeneous, fissile layers or folia, and in this case the rock is both foliated and has a cleavage.By STRATIFICATION, as applied to these formations, I mean those alternate, parallel, large masses of different composition, which are themselves frequently either foliated or fissile,--such as the alternating so-called strata of mica-slate, gneiss, glossy clay-slate, and marble.

The folia of the gneiss within a few miles round Bahia generally strike irregularly, and are often curvilinear, dipping in all directions at various angles: but where best defined, they extended most frequently in a N.E.by N.(or East 50 degrees N.) and S.W.by S.line, corresponding nearly with the coast-line northwards of the bay.I may add that Mr.

Gardner found in several parts of the province of Ceara, which lies between four and five hundred miles north of Bahia, gneiss with the folia extending E.45 degrees N.; and in Guyana according to Sir R.Schomburgk, the same rock strikes E.57 degrees N.Again, Humboldt describes the gneiss-granite over an immense area in Venezuela and even in Colombia, as striking E.50degrees N., and dipping to the N.W.at an angle of fifty degrees.(Gardner "Geological Section of the British Association" 1840.For Sir R.