Complex sentence: Hart, Gary,'James Monroe' (2005), p.52 Monroe resigned as Secretary of State on October 1, but no successor was ever appointed and thus from October 1,1814 to February 28,1815, Monroe effectively held both Cabinet posts.
Split into simpler sentences: Hart, Gary,'James Monroe' (2005), p.52 Monroe resigned as Secretary of State on October 1, but no successor was ever appointed, so he continued doing the work. Thus from October 1,1814, to February 28,1815, Monroe effectively held both cabinet posts.