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Plagioclase, microcline and quartz are the predominating minerals, while biotite, titanite, epidote, apatite, zircon and garnet are present in smaller quantities. There is also a small amount of hematite, pyroxene and sericite. The rock, which is of a granitic composition, contains numerous crystallites and has been subjected to considerable strain and crushing, which has resulted in foliation.
In the gneiss round Valparaiso the strike of the foliation is very variable, but I think about N. by W. and S. by E. is the commonest direction; this likewise holds good with the cleavage of the altered feldspathic clay-slates, occasionally met with on the coast for ninety miles north of Valparaiso.
In a letter to Mr. 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.
Similar instances abound in old pictures. The foliation of German work was generally crisp and full of convolutions in its minor features, though the leading lines were boldly conceived. We give an example from a panel carved in wood, in the Cathedral of Stuttgard, a work of the middle of the fifteenth century. It is almost a return to the old acanthus leaf, and so completes a cycle of fine art.
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.
D. Forbes has pointed out a striking case where the foliation is identical with the lines of stratification in rocks well seen near Crianlorich on the road to Tyndrum, about eight miles from Inverarnon, in Perthshire.
Seeing, then, that foliated schists indisputably are sometimes produced by the metamorphosis of homogeneous fissile rocks; seeing that foliation and cleavage are so closely analogous in the several above-enumerated respects; seeing that some fissile and almost homogeneous rocks show incipient mineralogical changes along the planes of their cleavage, and that other rocks with a fissile structure alternate with, and pass into varieties with a foliated structure, I cannot doubt that in most cases foliation and cleavage are parts of the same process: in cleavage there being only an incipient separation of the constituent minerals; in foliation a much more complete separation and crystallisation.
There is one interesting exception to the uniformity in the strike of the foliation. On the broad summit, as far as the southern crest, there is much mica-slate, in some places even 400 feet in thickness, with the folia all dipping north, at angles varying from 5 degrees to 20 degrees, but sometimes mounting up to 30 degrees. The southern flank consists of bare granite.
Near M. Video, the space which I more accurately examined was about fifteen miles in an east and west line, and here I found the foliation of the gneiss and the cleavage of the slates generally well developed, and extending parallel to the alternating strata composed of the gneiss, hornblendic and chloritic schists.
At Low's Harbour, however, a set of great parallel dikes, one ninety yards and another sixty yards in width, have been guided by the foliation of the mica-schist, and hence are inclined westward at an angle of 45 degrees: these dikes are formed of various porphyritic traps, some of which are remarkable from containing numerous rounded grains of quartz.
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