A.T. Robertson Commentary Acts 7:30

"And when forty years were fulfilled, an angel appeared to him in the wilderness of Mount Sinai, in a flame of fire in a bush." — Acts 7:30 (ASV)

Sentence begins with genitive absolute again.

In a flame of fire in a bush (εν φλογ πυρος βατου). 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, πυρος and βατου dependent on φλογ, flame). Descriptive genitives as in 9:15; 2 Thessalonians 1:8. Βατος (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.