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Updated: June 26, 2025
The Karuma Falls, if such they may be called, are a mere sluice or rush of water between high syenitic stones, falling in a long slope down a ten-feet drop. There are others of minor importance, and one within ear-sound, down the river, said to be very grand.
Sandstone overlying coal; trap dykes; syenitic porphyry dykes; black vesicular trap, penetrating in thin veins the clay shale of the country, converting it into porcellanite, and partially crystallizing the coal. On this sandstone lie fossil palms, and coniferous trees converted into silica, as on a similar rock in Angola. IGNEOUS ROCKS. Trappean rocks, with hot fountain.
This change of direction gave me occasion to trace the map of the Rio Magdalena, to determine astronomically the position of eighty points situated in the inland country between Carthagena, Popayan, and the upper course of the river Amazon and Lima, to discover the error in the longitude of Quito, to collect several thousand new plants, and to observe on a vast scale the relations between the rocks of syenitic porphyry and trachyte with the fire of volcanoes.
At the height of land dividing the waters that flow in different directions, into Esquimaux and Ungava Bays, the formation becomes syenitic schist, and continues so to within a short distance of the great fall on Hamilton River; when syenite succeeds; then gneiss; and along the shores of Esquimaux Bay syenitic gneiss, and pure quartz: lumps of black and red hornblend are met with everywhere.
Here, for the first time, we saw hills with bare, smooth, rocky tops, and we crossed over broad dikes of gneiss and syenitic porphyry: the directions in which they lay were N. and S. As we were now near to Tete, we were congratulating ourselves on having avoided those who would only have plagued us; but next morning some men saw us, and ran off to inform the neighboring villages of our passing.
The granites of the Orinoco, it is true, often contain hornblende; and those who are accustomed to practical labour in mines are not ignorant that the most noxious exhalations rise from galleries wrought in syenitic and hornblende rocks: but in an atmosphere renewed every instant by the action of little currents of air, the effect cannot be the same as in a mine.
Here they are of three kinds: coarse and rough basaltic lava for the first and rudest work; red granite and syenitic granite for the next stage; and, lastly, an admirable handmill of the compactest grey granite, smooth as glass and hard as iron.
The secondary formations on the east of the Havannah are pierced in a singular manner by syenitic and euphotide rocks united in groups. Going from north to south, and first near Marimelena, we find syenite consisting of a great quantity of hornblende, partly decomposed, a little quartz, and a reddish-white feldspar seldom crystallized.
The latitude of Mandavaca is 2 degrees 4 minutes 7 seconds; its longitude, according to the chronometer, 69 degrees 27 minutes. Yet the surrounding rocks are of the same granite, mixed with a little hornblende, which we had found at Javita, and which assumes a syenitic aspect. We left Mandavaca at half-past two in the morning.
Decomposing into Spherical Masses. Rude columnar Structure. Graphic Granite. Mutual Penetration of Crystals of Quartz and Feldspar. Glass Cavities in Quartz of Granite. Porphyritic, talcose, and syenitic Granite. Schorlrock and Eurite. Syenite. Connection of the Granites and Syenites with the Volcanic Rocks. Analogy in Composition of Trachyte and Granite.
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