Although rocks wholly or in large part vitreous are known from very ancient geological systems, such as the Devonian, they are certainly most frequent in recent volcanic countries. Yet among the older rocks there are many which, though finely crystalline, have the chemical composition of modern obsidians and possess structures, such as the perlitic and spherulitic, which are very characteristic of vitreous rocks. By many lines of evidence we are led to believe that obsidians in course of time suffer devitrification, in other words they pass from the vitreous into a crystalline state, hut as the changes take place in a solid mass they require a very long time for their achievement, and the crystals produced are only of extremely small size. A dull stony-looking rock results, the vitreous lustre having entirely disappeared, and in microscopic section this exhibits a cryptocrystalline structure, being made up of exceedingly minute grains principally of quartz and felspar. Often this felsitic devitrified glass is so fine-grained that its con stituents cannot be directly determined even with the aid of the microscope, but chemical analysis leaves little doubt as to the real nature of the minerals which have been formed. Many vitreous rocks show alteration of this type in certain parts where either the glass has been of unstable nature or where agencies of change such as percolating water have had easiest access (as along joints, perlitic cracks and the margins of dikes and sills). Obsidians from
Lipari often have felsitic bands alternating with others which are purely glassy. In Arran there are pitchstone dikes, some of which are very completely vitreous, while others are changed to spheru litic felsites more or less silicified.
Obsidians occur in many parts of the world along with rhyolites and pumice. In Europe the best-known localities for them are the Lipari Islands, Pantellaria, Iceland and Hungary. Very fine obsi dians are also obtained in Mexico, at the Yellowstone Park, in New Zealand, Ascension and in the Caucasus. Included in this group are some rocks which are more properly to be regarded as vitreous forms of trachyte than as glassy rhyolites (Iceland), but except by chemical analyses they cannot be separated. It is certain, how ever, that most obsidians are very acid or rhyolitic. The dark, semi-opaque glassy forms of the basic igneous rocks are known as tachylytes. The typical obsidians exhibit the chemical peculiari ties of the acid igneous rocks (viz., high percentage of silica, low iron, lime and magnesia, and a considerable amount of potash and soda) as shown in the following table.
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