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Updated: June 13, 2025
In common sand, the quartz grains are the most numerous; but this is not a proof that the rocks from which these particles were derived were wholly, or even chiefly, quartzose in character; for, in many composite rocks, as, for example, in the granitic group, the mica, feldspar, and hornblende are more easily decomposed by chemical action, or disintegrated, comminuted, and reduced to an impalpable state by mechanical force, than the quartz.
Northward of Las Minas, the more regular northerly ranges predominate: from this place to near Polanco, we meet with the coarse-grained mixture of quartz and feldspar, often with the imperfect hornblende, and then becoming foliated in a N. and S. line with imperfect clay-slate, including laminae of red crystallised feldspar with white or black marble, sometimes containing asbestus and crystals of gypsum with quartz-rock with syenite and lastly, with much granite.
The prevailing rock is gneiss, often passing, by the disappearance of the quartz and mica, and by the feldspar losing its red colour, into a brilliantly grey primitive greenstone. Not unfrequently quartz and hornblende are arranged in layers in almost amorphous feldspar.
5-Mica Granite Gneiss Country Rock near Caribou Ridge. In the hand specimen the rock has the same appearance as No. 4, if anything, it is somewhat more compact. The principal minerals are, plagioclase, biotite and microcline, with smaller quantities of quartz, iron oxide, pyroxene and garnet. The feldspar is decomposed with the resulting formation of epidote, which is quite prominent.
The heat must have been indeed intense which could cause mica to disappear entirely, and feldspar to melt almost completely. The hill known as Ord Hill of Kissock is crowned, as is Craig Phoedrick, with ruins still standing, but the vegetation about them is so dense and thorny that it is difficult to make out the condition of the remains.
It should not, however, be overlooked, that all the strata here have undergone metamorphic action, which may have caused crystals of feldspar to appear, and other changes to be effected, in the originally simple amygdaloidal balls. Mr.
I never saw in Europe such fine hyalites as I found in the island of Graciosa, and on the rock of porphyry called el Penol de los Banos, on the bank of the lake of Mexico. Two kinds of sand cover the shore; one is black and basaltic, the other white and quartzose. The quartzose sand contains fragments of feldspar.
We saw between Muitaco and the island of Ceiba a hill entirely composed of balls with concentric layers, in which we perceived a close mixture of hornblende and feldspar, with some traces of pyrites. The grunstein resembles that in the vicinity of Caracas; but it was impossible to ascertain the position of a formation which appeared to me to be of the same age as the granite of Parima.
The rock has been subjected to strong shearing force, producing a good example of schistose structure. 12 Quartzite Same location as No. 9. A compact rock of light red color, made up of uniformly rounded grains of quartz, and the feldspar with occasional grain of magnetite. A fine siliceous material discolored by iron oxide, acts as a cement between the grains.
Ordinary granite, as well as syenite and eurite, usually contains two kinds of feldspar: First, the common, or orthoclase, in which potash is the prevailing alkali, and this generally occurs in large crystals of a white or flesh colour; and secondly, feldspar in smaller crystals, in which soda predominates, usually of a dead white or spotted, and striated like albite, but not the same in composition.
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