The structures of lavas, however, furnish their most easily appreciated characteristics. Four of these structures deserve more particular attention: 1st, Cellular, vesicular or pumiceous structure; 2nd, The presence of glass, or some result of the devitrification of an original glass; 3rd, Flow-structure; and 4th, The arrangement of the rocks in sheets or beds, with columnar and other structures. [Illustration: Fig. 1.--Vesicular structure, Lava from Ascension Island, slightly less than natural size.] 1. The CELLULAR, VESICULAR, SCORIACEOUS or PUMICEOUS STRUCTURE of volcanic rocks (Fig. 1) could only have arisen in molten masses from the expansion of imprisoned vapours or gases, and is thus of crucial importance in deciding the once liquid condition of the rocks which display it. The vesicles may be of microscopic minuteness, but are generally quite visible to the naked eye, and are often large and conspicuous. Sometimes these cavities have been subsequently filled up with calcite, quartz, agate, zeolites or other mineral deposition. As the kernels thus produced are frequently flattened or almond-shaped (_amygdales_), owing to elongation of the steam-holes by movement of the lava before its consolidation, the rocks containing them are said to be _amygdaloidal_. This structure, though eminently characteristic of superficial lavas, is not always by itself sufficient to distinguish them from the intrusive rocks. Examples will be given in later chapters where dykes, sills and other masses of injected igneous material are conspicuously cellular in some parts. But, in such cases, the cavities are generally comparatively small, usually spherical or approximately so, tolerably uniform in size and distribution, and, especially when they occur in dykes, distributed more particularly along certain lines or bands, sometimes with considerable regularity (see Figs. 90, 91, and 236).