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Updated: June 7, 2025
Geikie has since shown that there is a strong probability that this Plutonic rock may be of Miocene age, because a similar Syenite having a true granitic character in its crystallisation has modified the Tertiary volcanic rocks of Ben More, in Mull, some of which have undergone considerable metamorphism.
They exhibit great variety of decoration and no little care in finish. The northern family shows a dark paste tempered with sand, often apparently granitic; a rough fracture, and generally a rude finish. The shapes are comparatively simple, often long, tapering below, and flat bottomed. The ornamentation is totally unlike that of the southern variety.
This latter rock was in one spot remarkable from being full of drusy cavities, lined with quartz crystals, and arranged in planes, dipping at an angle of 50 degrees to the east, and striking parallel to the foliation of an adjoining hill composed of the common mixture of quartz, feldspar, and imperfect hornblende: this fact perhaps indicates that these volcanic rocks have been metamorphosed, and their constituent parts rearranged, at the same time and according to the same laws, with the granitic and metamorphic formations of this whole region.
If the balls rested on a rock of a different nature, as in the blocks of Jura, we might suppose that they had been rounded by the action of water, or thrown out by the force of an elastic fluid; but their position on the summit of a hill alike granitic, makes it more probable that they owe their origin to the progressive decomposition of the rock.
Fifty years ago, it was thought that all granitic rocks were primitive, or existed before any sedimentary strata; but it is now "no easy task to point out a single mass of granite demonstrably more ancient than all the known fossiliferous deposits."
It is a vast granitic plain, in which from league to league the rock pierces the soil, and forms, not hillocks, but small masses, that resemble pillars or ruined buildings. On the 1st of May the Indians chose to depart long before sunrise. In those humid regions covered with forests, the nights became more obscure in proportion as we drew nearer to the Rio Negro and the interior of Brazil.
There were only three of us: we were completely wet, and uneasy respecting the fate of our boat: it appeared far from agreeable to pass, without sleep, a long night of the torrid zone amid the noise of the Raudales. M. Bonpland proposed to leave me in the island with Don Nicolas Soto, and to swim across the branches of the river that are separated by the granitic dikes.
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.
Having finished our dinner, of hail-stricken meat, we crossed the Sierra Tapalguen; a low range of hills, a few hundred feet in height, which commences at Cape Corrientes. The rock in this part is pure quartz; further eastward I understand it is granitic.
In the old granitic districts, no doubt it would be rash to attribute all the modifications of outline exclusively to the sea-action; for who can say how often this lately submerged coast may not previously have existed as land, worn by running streams and washed by rain? This source of doubt, however, does not apply to the districts superficially formed of the modern tertiary deposits.
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