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Updated: June 12, 2025
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.
This free quartz is in some cases found to constitute large irregular crystalline grains in the mass, just like those of the ordinary orthoclase quartz-trachytes; but at other times its presence can only be detected by the microscope in thin sections.
In the hand specimen it is an apparently pure orthoclase but in the thin section small scattered quartz grains are observed; as well as the alteration products, Kaolin and sericite. The minerals at contact are quartz, biotite, magnetite and hornblende. Both the quartz and orthoclase contain dust inclusions and crystallites, while the evidences of shearing and crushing are abundant.
It had been argued that the quartz in granite having a specific gravity of 2.6, was not of purely igneous origin, because the silica resulting from fusion in the laboratory has only a specific gravity of 2.3. But Mr. Orthoclase, often called common or potash- feldspar. 2.
Under the microscope it is a confused mass of finely granulated minerals, with numerous included crystals. The rock has undergone complete metamorphism and its origin is unknown. 8 Phyllite-Near Otter Lake. A soft extremely fine grained gray rock, with a well developed schistose structure, carrying much magnetite, plagioclase, orthoclase and their alteration products.
Orthoclase is orthoclastic and monoclinic; all the other feldspars are plagioclastic and triclinic. That variety of the Feldspar Group which is called Anorthite has been shown by Rammelsberg to occur in a meteoric stone, and his analysis proves it to be almost identical in its chemical proportions to the same mineral in the lavas of modern volcanoes.
Tridymite: 100.0 2.3. Orthoclase. Orthoclase. Albite. Oligoclase. Oligoclase. Labradorite. Labradorite. Anorthite. Anorthite. Leucite. Nepheline. Nepheline. Muscovite. Lepidolite. Li 4.85. Biotite. Biotite. Phlogopite. Margarite. Chlorite. Rapidolite. Talc. 63.00 .... .... trace .... 33.60 .... .... W 3.10 2.78. Tremolite. St. Actinolite. Hornblende. Uralite. Augite. Augite. Diallage.
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