Sentence begins with genitive absolute again.

In a flame of fire in a bush (εν φλογ πυρος βατου). Horeb in Exodus 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, πυρος and βατου dependent on φλογ, flame). Descriptive genitives as in Acts 9:15; 2. Thessalonians 1:8. Βατος (bush) is the wild acacia (mimosa nilotica). In Exodus 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.