68 ELEMENTARY GEOLOGY 
dules, and the rock an amygdaloid (Fig. 24). Calcite 
is one of the most common minerals in these cavities, 
but there are many others which are sometimes present. 
The rough surface of a lava flow or of a lava rock, 
caused by the expansion of steam, is sometimes so full 
FiG,-23, 
Spherulites in an obsidian rock from Yellowstone Park. 
of gas cavities that it is, in this respect, almost like a 
voleanic ash. It is then called a pumiceous or scoria- 
ceous lava. <A slaggy lava is one which resembles com- 
mon furnace slag; and: some of this is so like lava 
that it might be confused with it unless carefully 
examined. 
