(1) Among the acid rocks, what is called the Felsitic type of devitrification is characteristic. Thus, obsidians pass by intermediate stages from a clear transparent or translucent glass into a dull flinty or horny mass. When thin slices of these transitional forms are examined under the microscope, minute hairs and fibres or trichites, which may be observed even in the most perfectly glassy rocks, are seen to increase in number until they entirely take the place of the glass. Microlites of definite minerals may likewise be observed, together with indefinite granules, and the rock finally becomes a rhyolite, felsite or allied variety (Fig. 3). [Illustration: Fig. 3.--Microlites of the Pitchstone of Arran (magnified 70 diameters).] At the same time it should be observed that, even in the vitreous condition of a lava, definite crystals of an early consolidation were generally already present. Felspars and quartz, usually in large porphyritic forms, may be seen in the glass, often so corroded as to indicate that they were in course of being dissolved in the magma at the time of the cooling and solidification of the mass. In obsidians and pitchstones such relics of an earlier or derived series of crystallized minerals may often be recognized, while in felsites and quartz-porphyries they are equally prominent. Where large dispersed crystals form a prominent characteristic in a rock they give rise to what is termed the _Porphyritic_ structure. [Illustration: Fig. 4.--Perlitic structure in Felsitic Glass, Isle of Mull (magnified).] [Illustration: Fig. 5.--Spherulitic structure (magnified).] Accompanying the passage of glass into stone, various structures make their appearance, sometimes distinctly visible to the naked eye, at other times only perceptible with the aid of the microscope. One of these structures, known as _Perlitic_ (Fig. 4), consists in the formation of minute curved or straight cracks between which the vitreous or felsitic substance, during its contraction in cooling, assumed a finely globular form. Another structure, termed _Spherulitic_ (Fig. 5), shows the development of globules or spherules which may range from grains of microscopic minuteness up to balls two inches or more in diameter. These not infrequently present a well-formed internal fibrous radiation, which gives a black cross between crossed Nicol prisms. Spherulites are more especially developed along the margins of intrusive rocks, and may be found in dykes, sills and bosses (see Figs. 375 and 377). Where the injected mass is not thick it may be spherulitic to the very centre, as can be seen among the felsitic and granophyric dykes of Skye.