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Updated: May 6, 2025
Decomposing into Spherical Masses. Rude columnar Structure. Graphic Granite. Mutual Penetration of Crystals of Quartz and Feldspar. Glass Cavities in Quartz of Granite. Porphyritic, talcose, and syenitic Granite. Schorlrock and Eurite. Syenite. Connection of the Granites and Syenites with the Volcanic Rocks. Analogy in Composition of Trachyte and Granite.
The different varieties of rock allied to gneiss, into which feldspar enters as an essential ingredient, will be understood by referring to what was said of granite. EURITE, which has already been mentioned as a Plutonic rock, occurs also with precisely the same composition in beds subordinate to gneiss or mica-slate.
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.
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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