When it cools it becomes hard, but if before it is quite cold we plunge it into cold water a very perfect perlitic structure will arise in it.
If the matrix, however, is originally crystalline it does not seem probable that perlitic structure can develop in it.
They have long been known to geologists and are found at Okhotsk, Siberia, in association with a large mass of perlitic obsidian.
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.
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).
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