I could not find the 42nd Ms. Cavalry Regiment, but later I did find an article that stated the 2nd Cavalry Regiment was also called 4th and 42nd Regiment and that it was organized in 1863. I never could find Tomas A. Hall, but an ancestor of his told me that he was in the 42nd. Ms. Cavalry and apparently, they were wrong.
Thanks for all your help. Annelle