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Updated: July 6, 2025
Epidote likewise occurs in the gneiss in thin layers, parallel to the foliation of the mass. One large vein of a coarse granitic character was remarkable from in one part quite changing its character, and insensibly passing into a blackish porphyry, including acicular crystals of glassy feldspar and of hornblende: I have never seen any other such case.
I observed a fine sienite on several spots; it is of a whitish colour, and contains hornblende and mica in almost equal quantities; granite was also seen, and both rocks probably belong to each other, the presence of hornblende being local. A very hard pudding-stone crops out about nine miles down the river. From the ridges, hills were seen to the N.N.E. and to the westward.
Trachyte, which consists chiefly of feldspar, with some hornblende and oxide of iron, has a specific gravity of about 2.45; whilst basalt, composed chiefly of augite and feldspar, often with much iron and olivine, has a gravity of about 3.0. This order of eruption has been observed by Beudant, Scrope, and by other authors; three instances, also, have been given in this volume.
The failure of the attempts made under the intendant, Don Jose Avalo, must be attributed solely to the ignorance of the persons employed by the Spanish government who mistook mica and hornblende for metallic substances.
It also passes in the same uninterrupted manner into a basalt, and at length into a soft claystone, with a schistose tendency on exposure, in no respect differing from those of the trap islands of the western coast." The same author mentions, that in Shetland a granite composed of hornblende, mica, feldspar, and quartz graduates in an equally perfect manner into basalt.
Taking a general view of the primitive formation of Atures, we perceive, that, like the granite of Syene in Egypt, it is a granite with hornblende, and not a real syenite formation. Many of the layers are entirely destitute of hornblende. The black crust is 0.3 of a line in thickness; it is found chiefly on the quartzose parts.
Its texture becomes granular; the mica, less frequent, is more unequally spread through the rock. Instead of garnets we met with a few solitary crystals of hornblende. It is, however, not a syenite, but rather a granite of new formation. We were three quarters of an hour in reaching the summit of the pyramid.
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.
Their rusty brown or yellowish colour is partly due to the oxides of iron, but chiefly to innumerable, microscopically minute, black specks, which, when a fragment is heated, are easily fused, and evidently are either hornblende or augite.
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.
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