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Updated: June 13, 2025
Close to where we sat were the great masses of rock that had fallen from the cliffs, and upon examination I found them to be the finest quality of grey granite, the feldspar being in masses several inches square and as hard as a flint. There was no scaling upon the surface, as is common in granite rocks.
The strata of the serpentine, where it is stratified with some regularity, run hor. 8, and dip almost perpendicularly. Near the village of San Juan, the granular diabasis appears alone uncovered, and takes a greenish black hue. The feldspar intimately mixed with the mass, may be separated into distinct crystals. The mica is very rare, and there is no quartz.
When within about eight feet of the next shelf, I twisted myself round upon the face, hanging by two rough blocks of protruding feldspar, and looked vainly for some further hand-hold; but the rock, besides being perfectly smooth, overhung slightly, and my legs dangled in the air.
The kaolin, or clay, used in most countries for the manufacture of fine porcelain or china, is generally produced from the feldspar of decomposing granite, in which the cause of decay is the dissolution and separation of the alkaline ingredients. Eub.
The coarse granites and gneisses proclaim still more clearly that they must have originated far down in the depths of the earth; their huge crystals of mica, quartz, hornblende, feldspar, and other minerals could never have been formed except under a blanket of rock which almost prevented the original magmas from cooling.
And he had been fool enough to cart it up four flights of stairs! He turned the piece of stone over in his hands. It was of porphyritic granite, with distinct crystals of feldspar embedded in a fine grained matrix. Trotter's brow wrinkled in vague thought as he peered down at it. He was trying to think what it reminded him of, what possible link it made in a chain of lost association.
The rock was composed principally of quartz and feldspar. A little below it, we found a large tribe anxiously awaiting our arrival. They crowded to the margin of the river with great eagerness, and evinced more surprise at our appearance than any tribe we had seen during the journey; but we left them very soon, notwithstanding that they importuned us much to stay.
None of these rocks contain large crystals of feldspar, or have the harsh fracture peculiar to trachyte. These feldspathic lavas and tuffs are the uppermost or those last erupted; innumerable dikes, however, and great masses of molten rock, have subsequently been injected into them.
When feldspar and mica are also present, it may be called schorly granite. This kind of granite is comparatively rare. Eurite is a rock in which the ingredients of granite are blended into a finely granular mass, mica being usually absent, and, when present, in such minute flakes as to be invisible to the naked eye.
Feldspathic rocks which contain no iron compounds thus weather to whitish crusts, and even apparently sound crystals of feldspar, when ground to thin slices and placed under the microscope, may be seen to be milky in color throughout because an internal change to kaolin has begun.
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