THE IGNEOUS OR ZRUPTIVE ROCKS 67 
and planes; but at times the boundaries are rounded 
and eaten by the hot lava in which they grew. They 
were developed while the rock was cooling from the 
fluid state. 
More rarely some of the minerals form balls, known 
9 
as spherulites (Fig. 23), in which the minerals radiate 
Pie 22: 
Photograph of a porphyritic rock with crystals of feldspar in a black, erypto- 
crystalline groundmass. 
from a centre like the spokes of a wheel. Many lavas 
have numerous cavities where steam has expanded while 
the rock was cooling (Fig. 15); and if, later, these are 
filled with minerals, the filled cavities are called amyg- 
