Heat was produced either by a rubbing or drilling action between two pieces of wood, or by the less frquently observed percussion method of producing sparks when two pieces of stone were struck together.
(Heat; was produced; either by a rubbing or drilling action between two pieces of wood, or by the less frquently observed percussion method of producing sparks) (sparks; were struck together; two pieces of stone)
Heat sparks
was produced were struck together
either by a rubbing or drilling action between two pieces of wood, or by the less frquently observed percussion method of producing sparks
two pieces of stone