chap, m 
STRUCTURES OF LAVAS 
15 
Tile structures of lavas, however, furnish their most easil)’ appreciated 
characteristics. Four of these structures deserve more particular attention : 
Istj Cellular, vesicular or pumiceous structure ; 2nd, The presence ot glass, or 
some result of the devitrification of an original glass ; Mrd, h low-structure ; 
^od 4th, The arrangement of the rocks in sheets or beds, with columnar and 
ether structures. 
1. The CELLULAK, VESICULAR, SCORIACEOUS Or PUMICEOUS STRUCTURE of 
''olcanic rocks (Fig. 1) could only have arisen in molten masses from the 
Pig. I. — Ve.sicul.ir structure, Lava front Ascension Isltind, slightly less than natural size. 
expansion of imprisoned vapours or gases, and is thus of crucittl importance 
deciding the once liquid condition of the rocks which display it. The 
^esicles may Ite of microscopic minuteness, Imt are generally quite visible 
° the naked eye, and are often large and conspicuous. Sometimes these 
'^9'Vities have been subsequently tilled up with calcite, quartz, agate, zeolites 
a *^ther mineral deposition. As the kernels thus produced tire frequently 
eiied or almond-shaped (ainj/f/dales), owing to elongation of the steam- 
o es by movement of the lava hefoi’e its consolidation, the rocks containing 
®i^i^are said to he aniyfjdaloidal. 
This structure, though eminently characteristic of superficial lavas, is 
