Sentence begins with genitive absolute again. {In a flame of
fire in a bush} (en phlogi puros batou). Horeb in  Ex 3:1 ; but
Sinai and Horeb were "probably peaks of one mountain range"
(Page), Horeb "the mountain of the dried-up ground," Sinai "the
mountain of the thorns." Literally, "in the flame of fire of a
bush" (two genitives, puros and atou dependent on phlogi,
flame). Descriptive genitives as in  9:15  2Th 1:8 . Batos
(bush) is the wild acacia (_mimosa nilotica_). In  Ex 3:20  it is
Jehovah who speaks. Hence "angel" here with Stephen is understood
to be the Angel of the Presence, the Eternal Logos of the Father,
the Angel of Jehovah.