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Updated: October 6, 2025
The white chalkstone glitters from afar; the light grey feldspar assumes a warm flesh tint; the limestone becomes straw color; the crystals of hornblende flash like fire-flies; and the veins of white quartz, running with their nodules of serpentine and chlorite through the dark clay-slate, gleam as do chain lightnings through the clouds.
Is it not more probable, that these dikes have been formed by fissures penetrating into partially cooled rocks of the granitic and metamorphic series, and by their more fluid parts, consisting chiefly of hornblende, oozing out, and being sucked into such fissures? Dr.
HORNBLENDE-SCHIST is usually black, and composed principally of hornblende, with a variable quantity of feldspar, and sometimes grains of quartz. When the hornblende and feldspar are in nearly equal quantities, and the rock is not slaty, it corresponds in character with the greenstones of the trap family, and has been called "primitive greenstone." It may be termed hornblende rock, or amphibolite.
There was something striking about him, thought Mr. Heard. He said little of consequence, but Denis listened enthusiastically to his abstruse remarks about fractures and so forth, and watched with eagerness as he poked his stick into the rough walls to dislodge some stone that seemed to be of interest. "So you don't know the difference between augite and hornblende?" he once enquired. "Really?
The little fragment which I procured indicated a stratified rock, rich in white feldspar, and containing, together with spangles of mica, grouped in streaks, and variously twisted, some crystals of hornblende. Beyond the confluence of the Macava, the Orinoco suddenly diminishes in breadth and depth, becoming extremely sinuous, like an Alpine torrent.
It must be observed that all these phenomena of coloration have hitherto appeared in the torrid zone only, in rivers that have periodical overflowings, of which the habitual temperature is from twenty-four to twenty-eight centesimal degrees, and which flow, not over gritstone or calcareous rocks, but over granite, gneiss, and hornblende rocks.
Mica schist crowned some of the heights on the watershed, then gneiss, and now, as we descend further, we have igneous rocks of more recent eruption, porphyry and gneiss, with hornblende.
Nearer the granite the schist often contains crystals of hornblende, which are even met with in some places for a distance of several hundred yards from the junction; and this black hornblende is so abundant that eminent geologists, when passing through the country, have confounded it with the ancient hornblende-schist, subordinate to the great gneiss formation of Norway.
Near the Lake, and along its eastern shore, we have mica schist and gneiss foliated, with a great deal of hornblende; but the most remarkable feature of it is that the rocks are all tilted on edge, or slightly inclined to the Lake. The active agent in effecting this is not visible. It looks as if a sudden rent had been made, so as to form the Lake, and tilt all these rocks nearly over.
In some places the siliceous matter of the schist becomes a granular quartz; and when hornblende and mica are added, the altered rock loses its stratification, and passes into a kind of granite.
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