第91章 LUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIA

Benza describes granitic rock, "Madras Journal of Literature" etc.October 183? page 246), in the Neelgherries, decomposed to a depth of forty feet.)The minerals retain their positions in folia ranging in the usual direction; and fractured quartz veins may be traced from the solid rock, running for some distance into the softened, mottled, highly coloured, argillaceous mass.It is said that these decomposed rocks abound with gems of various kinds, often in a fractured state, owing, as some have supposed, to the collapse of geodes, and that they contain gold and diamonds.At Rio, it appeared to me that the gneiss had been softened before the excavation (no doubt by the sea) of the existing, broad, flat-bottomed valleys; for the depth of decomposition did not appear at all conformable with the present undulations of the surface.The porphyritic gneiss, where now exposed to the air, seems to withstand decomposition remarkably well; and Icould see no signs of any tendency to the production of argillaceous masses like those here described.I was also struck with the fact, that where a bare surface of this rock sloped into one of the quiet bays, there were no marks of erosion at the level of the water, and the parts both beneath and above it preserved a uniform curve.At Bahia, the gneiss rocks are similarly decomposed, with the upper parts insensibly losing their foliation, and passing, without any distinct line of separation, into a bright red argillaceous earth, including partially rounded fragments of quartz and granite.From this circumstance, and from the rocks appearing to have suffered decomposition before the excavation of the valleys, I suspect that here, as at Rio, the decomposition took place under the sea.The subject appeared to me a curious one, and would probably well repay careful examination by an able mineralogist.

THE NORTHERN PROVINCES OF LA PLATA.

According to some observations communicated to me by Mr.Fox, the coast from Rio de Janeiro to the mouth of the Plata seems everywhere to be granitic, with a few trappean dikes.At Port Alegre, near the boundary of Brazil, there are porphyries and diorites.(M.Isabelle "Voyage a Buenos Ayres" page 479.) At the mouth of the Plata, I examined the country for twenty-five miles west, and for about seventy miles north of Maldonado:

near this town, there is some common gneiss, and much, in all parts of the country, of a coarse-grained mixture of quartz and reddish feldspar, often, however, assuming a little dark-green imperfect hornblende, and then immediately becoming foliated.The abrupt hillocks thus composed, as well as the highly inclined folia of the common varieties of gneiss, strike N.N.E.or a little more easterly, and S.S.W.Clay-slate is occasionally met with, and near the L.del Potrero, there is white marble, rendered fissile from the presence of hornblende, mica, and asbestus; the cleavage of these rocks and their stratification, that is the alternating masses thus composed, strike N.N.E.and S.S.W.like the foliated gneisses, and have an almost vertical dip.The Sierra Larga, a low range five miles west of Maldonado, consists of quartzite, often ferruginous, having an arenaceous feel, and divided into excessively thin, almost vertical laminae or folia by microscopically minute scales, apparently of mica, and striking in the usual N.N.E.and S.S.W.direction.The range itself is formed of one principal line with some subordinate ones; and it extends with remarkable uniformity far northward (it is said even to the confines of Brazil), in the same line with the vertically ribboned quartz rock of which it is composed.The S.de Las Animas is the highest range in the country; Iestimated it at 1,000 feet; it runs north and south, and is formed of feldspathic porphyry; near its base there is a N.N.W.and S.S.E.ridge of a conglomerate in a highly porphyritic basis.

Northward of Maldonado, and south of Las Minas, there is an E.and W.hilly band of country, some miles in width, formed of siliceous clay-slate, with some quartz, rock, and limestone, having a tortuous irregular cleavage, generally ranging east and west.E.and S.E.of Las Minas there is a confused district of imperfect gneiss and laminated quartz, with the hills ranging in various directions, but with each separate hill generally running in the same line with the folia of the rocks of which it is composed: this confusion appears to have been caused by the intersection of the [E.and W.] and [N.N.E.and S.S.W.] strikes.Northward of Las Minas, the more regular northerly ranges predominate: from this place to near Polanco, we meet with the coarse-grained mixture of quartz and feldspar, often with the imperfect hornblende, and then becoming foliated in a N.and S.line--with imperfect clay-slate, including laminae of red crystallised feldspar--with white or black marble, sometimes containing asbestus and crystals of gypsum--with quartz-rock--with syenite--and lastly, with much granite.The marble and granite alternate repeatedly in apparently vertical masses: some miles northward of the Polanco, a wide district is said to be entirely composed of marble.It is remarkable, how rare mica is in the whole range of country north and westward of Maldonado.Throughout this district, the cleavage of the clay-slate and marble--the foliation of the gneiss and the quartz--the stratification or alternating masses of these several rocks--and the range of the hills, all coincide in direction; and although the country is only hilly, the planes of division are almost everywhere very highly inclined or vertical.