They are usually dark-colored rocks having a compact or felsitic texture.
Felsitic matter cannot, even with the highest powers of the microscope, be resolved into separate grains or particles; and it does not exhibit, except perhaps very indistinctly, the phenomenon of double refraction.
Glass is an unstable form of mineral matter; and every species of glass, including obsidian, tends with the lapse of time to become crystalline or stony, the amorphous changing to the felsitic structure.
Feldspathic rocks exhibiting the felsitic texture in whole or in part are known as felsites.
The texture varies from perfectly compact or felsitic to coarsely crystalline; averaging, however, less coarse than syenite and granite.
The typical felsite or petrosilex is composed entirely of felsitic matter, and is perfectly homogeneous, like flint or jasper, which it closely resembles in hardness and other physical characteristics.
Trachyte from Hungary; felsitic paste with crystals of hornblende and sanidine, and a little magnetite.
It consists of a light-greyish felsitic paste enclosing grains of smoke-quartz, crystals of sanidine, plagioclase and biotite, with a little magnetite and apatite.
In Thoka-singa we have therefore a hill which is evidently formed in mass of these holocrystalline felsitic andesites but covered in places with an agglomerate of the same materials.
Footnote 118: The term “felsitic andesite” is suitable for this microfelsitic type.
In the slide they display these pseudomorphs and quartz crystals, associated with abundant plagioclase phenocrysts, in a felsitic groundmass, evidently a mixture of felspar and quartz, with fine pyroxene, mostly prismatic and rhombic.
Footnote 91: These rocks are in most cases referred to the orthophyric and felsitic orders of the hypersthene-augite andesites.
As examples of thefelsitic order of these rocks, most of which are altered like the propylites, I will first take the case of those deep-seated rocks that are exposed in the river-bed above Nambuna in the Ndrandramea district.
Suggestion of “felsitic andesite” as a rock-name, 295.
They have a crypto-crystalline groundmass where thefelsitic structure is in process of development and where the pyroxene prisms or microliths are very minute.
In the slide they exhibit tabular phenocrysts of the plagioclase, together with dark pseudomorphs after hornblende, in a coarsely felsitic groundmass (grain ·022 mm.
The rocks exposed on the surface for the first mile are for the most part altered hypersthene-augite-andesites possessing a micro-felsitic groundmass.
The rocks of Na Raro and Vatu Kaisia differ as regards specific gravity, the “grain” of the felsitic groundmass, the presence of phenocrysts of rhombic pyroxene, etc.
The acid andesites, including the hornblende-andesites and the dacites or felsitic andesites, are best represented in the Ndrandramea district in the midst of the basic rocks.
The very great viscosity of acid magmas prevents their molecules, especially when cooling takes place suddenly, from arranging themselves to form discrete crystals, and is the principal cause of the production of felsitic ground-masses.
Felsitic rocks are sometimes silicified and have their matrix replaced by granular aggregates of cloudy quartz.
Some rocks are felsitic in parts but elsewhere glassy; and it is not always clear whether the felsite is an original substance or has arisen by the devitrification of primary glass.
The granitic mass of Priestlaw and other felsitic rocks have been intruded into these strata.
Halleflinta under the microscope is very finely crystalline, or even cryptocrystalline, resembling the felsitic matrix of many acid rocks.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "felsitic" in a variety of sentences. We hope that you will now be able to make sentences using this word.
