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THE JOURNAL OF GEOLOGY.
  1. If they are volcanic:
    1. They may appear to be striped, banded, or pseudo-"stratified" conformably to adjoining sedimentary deposits;
    2. They will probably be accompanied by fragmental (pyroclastic) material, which may or may not itself be really stratified. Such material will vary greatly in coarseness, containing bombs, agglomerates, breccias, tuffs, sands and ashes. The characteristics of these are:
      • 1) indiscriminate mixture of all sizes and shapes of fragments;
      • 2) material of same kind as the igneous rocks;
      • 3) cement, either finer fragmental material (tuff-breccia) or lava (flow-breccia);
      • 4) very angular shape of smallest fragments (microscopic glass sherds).
      • 5) if ancient volcanoes were on the shore-line, such material may have been immediately worked over by water and interbedded with more or less normal aqueous sediments.
  2. Most important of all, however, is the identification of those characteristic structures known to originate only in glassy, half-glassy or very fine grained porphyritic rocks, solidifying at the surface, or in very narrow dykes where solidification has been rapid. These will be found to be very persistent and can usually be identified under the microscope in spite of devitrification, alteration, or even a considerable degree of dynamometamorphism. The most common of these structures are:
    1. a vesicular, scoriaceous, pumiceous or amygdaloidal structure;
    2. a sharply defined, small porphyritic structure with a glassy, half-glassy or felsitic (cryptocrystalline) base;
    3. a spherulitic structure, due to either large or small lithopysæ, hollow spherulites, or compact spherulites,
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