Complex sentence: Seldon H. Gorham as acting head of the regiment as Col. Gordon Granger was elevated to lead the brigade that consisted of the 2nd Michigan and 3rd Michigan Cavalry.
Split into simpler sentences: Seldon H. Gorham was acting as head of the regiment as Col. Gordon Granger was elevated to lead the brigade. The brigade consisted of two regiments, the 2nd Michigan and 3rd Michigan Cavalry.