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3. arranged either irregularly, or in more or less discontinuous bands or layers;
4. a flow structure, produced either by the elongation of vesicles or the parallel arrangement of constituents or crystallites. It may also be produced by the interlacing of different colored magmas (eutaxitic structure);
5. corroded phenocrysts, quartz with embayments, or skeleton crystals due to rapid and imperfect growth;
6. microscopic spherulites, globulites, trichites, crystallites, real or devitrified glass inclusions, quartz with orientated siliceous aureoles, axiolites, etc.;
7. perlitic structure, wholly or partly devitrified.

Although some of these structures may occasionally occur in dykes or other igneous rocks which have rapidly solidified beneath the surface, they are nevertheless so essentially characteristic of effusive lavas, that, in lack of any evidence to the contrary, they may be regarded as fairly safe guides in establishing the effusive nature of rocks. This evidence is beyond doubt, if such rocks are accompanied, as they generally are, by ash material.

While a single one of these characteristics may not be sufficient to identify a volcanic occurrence, many, if not all of them, will be found to occur together, and only in rare instances will it be found that some of them, at least, have not survived the vicissitudes of metamorphism. That many regions in the ancient crystalline belt of the Appalachian system exhibit most of them in great perfection is now well known. It is only a misinterpretation of these characteristic features of volcanic rocks, due to a lack of acquaintance on the part of observers with their recent analogues, that has prevented their recognition long ago. Thus, by those who have heretofore described these rocks as sediments, both secondary cleavage, and the banding due to flow or parallel spherulitic layers have been mistaken for stratification; spherulites have been erroneously regarded as concretions; and the accompanying pyroclastics, as normal conglomerates or slates.