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Updated: June 13, 2025
We may believe that the same molten mass which supplies this acidic lava in surface flows solidifies as granite deep below ground in the volcanic reservoir. SYENITE, composed of feldspar and mica, has consolidated from a less siliceous mixture than has granite.
These brown-coloured, compact lavas, consist almost entirely of small glimmering scales, or of minute acicular crystals, of feldspar, placed close by the side of each other, and abounding with minute black specks, apparently of hornblende.
In the valley of Tia-auru, the commonest rocks are basalts with much olivine, and in some cases almost composed of large crystals of augite. I picked up some specimens, with much glassy feldspar, approaching in character to trachyte.
Where long- weathered granite forms the country rock, it often may be cut with spade or trowel for several feet from the surface, so rotten is the feldspar, and here the rock is seen to break down to a clayey soil containing grains of quartz and flakes of mica. These are a few simple illustrations of the surface changes which some of the common kinds of rocks undergo.
One of these, which is evidently the second in age, is seen to cut through an older granite; and another, still newer, traverses both the second and the first. In Shetland there are two kinds of granite. One of them, composed of hornblende, mica, feldspar, and quartz, is of a dark colour, and is seen underlying gneiss.
When it alternates with these rocks, it often contains some crystals of mica, and occasionally quartz, feldspar, hornblende, talc, chlorite, garnet, and other minerals. It enters sparingly into the structure of the hypogene districts of Norway, Sweden, and Scotland, but is largely developed in the Alps.
But in stones, much depends upon the peculiar nature of their constituent parts; when the feldspar of the granite rocks contains little alkali or calcareous earth, it is a very permanent stone; but, when in granite, porphyry, or sienite, either the feldspar contains much alkaline matter, or the mica, schorl, or hornblende much protoxide of iron, the action of water containing oxygen and carbonic acid on the ferruginous elements tends to produce the disintegration of the stone.
They contain vitreous feldspar, and augite, and are the same as the celebrated columnar porphyries of the island of Panaria, which, without having been part of a current of lava, seem raised up in the form of hillocks, like many of the porphyries in Auvergne, in the Euganean mountains, and in the Cordilleras of the Andes.
On the road to Catia we see the chloritic schist passing into hornblende schist. The sea at the foot of Cabo Blanco throws up on the beach rolled fragments of a rock, which is a granular mixture of hornblende and lamellar feldspar. It is what is rather vaguely called PRIMITIVE GRUNSTEIN. In it we can recognize traces of quartz and pyrites.
THIRDLY, a mass of confusedly crystallised white feldspar, with little nests of a dark-coloured mineral, often carious, externally rounded, having a glossy fracture, but no distinct cleavage: from comparison with the second specimen, I have no doubt that it is fused hornblende. He obtained two good cleavages of 86 degrees 30 minutes and 86 degrees 50 minutes.
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