The outer matrices of Thunder Eggs are usually composed of a form of this rock which surrounds the inner core often composed of opal or agate. This rock is characterized by phenocrysts of sanidine or alkali feldspar and a fine-grained groundmass of biotite and hornblende as well as plagioclase and quartz. A ghost town in Nevada shares its name with this rock, since it contains many local deposits of this extrusive rock, which is often pinkish in 
The outer matrices of Thunder Eggs are usually composed of a form of this rock which surrounds the inner core often composed of opal or agate. This rock is characterized by phenocrysts of sanidine or alkali feldspar and a fine-grained groundmass of biotite and hornblende as well as plagioclase and quartz. A ghost town in Nevada shares its name with this rock, since it contains many local deposits of this extrusive rock, which is often pinkish in hue and ►has a felsic composition. Commonly found at convergent boundaries and hot spots, FTP, name this extrusive equivalent of granite
The outer matrices of Thunder Eggs are usually composed of a form of this rock which surrounds the inner core often composed of opal or agate.
rhyolite