Xenoliths = Strange Rock
A xenolith is a one type of rock embedded in a different igneous rock (solidified magma) like chocolate chunks in cookie dough. Xenoliths range from sand size to boulders more than one foot in diameter. The Menan Butte cones contain xenoliths of basalt and of river gravel carried upward by the explosions in the vent.
Notice the shape of xenoliths. When the xenolith shape is angular or rough-edged with a dark color, they are fragments of basalt torn from the walls of the underground feeding conduits below the surface of where you are now standing (see below photo).
This angular xenolith is embedded into the volcanic tuff.
Rounded xenoliths are river rock that has been transported and rounded by the South Fork of the Snake River. These rocks are composed of granite, quartzite, and schist which are found in the Teton Range 50 miles from here.