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Updated: June 13, 2025
In places it is spotted with the serpentine, and judging from its rough state resembles "verde antique," and at that of a beautiful color; samples of this should be obtained. Feldspar. This mineral occurs very plentfully in the space between the limestones and gneiss.
The ground rose appreciably as it moved away from the sand flats by the waves, and we soon arrived at some long, winding gradients, genuinely steep paths that allowed us to climb little by little; but we had to tread cautiously in the midst of pudding stones that weren't cemented together, and our feet kept skidding on glassy trachyte, made of feldspar and quartz crystals.
There is a resemblance in such cases to the diagonal lamination which we see in sedimentary rocks, even though the layers of quartz and of mica, or of feldspar and other minerals, may be more distinct in alternating folia than they were originally. Difficulty of ascertaining the Age of metamorphic Strata. Metamorphic Strata of Eocene date in the Alps of Switzerland and Savoy.
This whole district is almost exclusively formed of gneiss, abounding with garnets, and porphyritic with large crystals, even three and four inches in length, of orthoclase feldspar: in these crystals mica and garnets are often enclosed. At the western base of the Corcovado, there is some ferruginous carious quartz-rock; and in the Tijeuka range, much fine- grained granite.
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.
Sometimes one will slip through, and you can see on the finished dish what a bad-looking place it makes. Even for the coarsest earthenware, such as flower-pots, the moist clay is forced down a cylinder and through a wire sieve; and for stoneware and porcelain it has to go through several processes. When flint and feldspar are used, they are ground fine at the quarry.
In the long line of cliffs, between fifty and sixty feet in height, called the Barrancas de S. Gregorio, which extend westward of the Rio S. Lucia, the lower half is formed of coarse sand of quartz and feldspar without mica, like that now cast up on the beach near Maldonado; and the upper half of Pampean mud, varying in colour and containing honeycombed veins of soft calcareous matter and small concretions of tosca-rock arranged in lines, and likewise a few pebbles of quartz.
In this base also appear black specks of mica, the crystals of which have a more or less perfect hexagonal outline. The remainder of the mass is quartz, the translucency of which is strongly contrasted to the opaqueness of the white feldspar and black mica. But neither the transparency of the quartz nor the silvery lustre of the mica can be expressed in the engraving.
As we have seen that crystals of feldspar, in the instance described by Von Buch, sink in obsidian, in accordance with their known greater specific gravity, we might expect to find in every trachytic district, where obsidian has flowed as lava, that it had proceeded from the upper or highest orifices.
The specimens of stone which I brought from the hills in the neighbourhood of Bencoolen were pronounced by some mineralogists, to whom I showed them at the time, to be granite; but upon more particular examination they appear to be a species of trap, consisting principally of feldspar and hornblende, of a greyish colour and nearly similar to the mountain stone of North Wales.
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