I've heard about the resignation but have no idea why....except maybe his agreed enlistment was up, he maybe did not like the idea of being combined with Missouri and/or Georgia units. I have a letter written by his wife, (my Great Grandmother), after the war, mentioning Capt.Tull, being wounded at the Battle of Pea Ridge (Elkhorn Tavern). After recovery he was promoted to Major. Shortly after his promotion he turns up in, Commerce,Texas, overseeing the mining of "Bat Dung" for use in artillery ammunition.
I have an original letter he wrote during the conflict describing their capture of a Yankee encampment, score of equipment, etc. .

After the war he apparently returned to his farm near Vicksburg,MS....then left for Galveston,Texas where he got a job as a Marine Draftsman. Shortly after that his wife joined him there where she opened a "Finishing School for Girls & Young Women". They remained in Galveston until after the 1900 Hurricane....when they moved to Houston,Texas.