John,
Texas was not a "solidly South" state.
To quote a passage from the book "Secession and the Union in Texas" by Walter L. Buenger, "One of the many fascinating things about secession in Texas was that wihtin this state, located on the Gulf of Mexico, an almost exact replica of a border state existed."
To quote Todd's AME, "Most Union sympathizers in Texas were found in the northern counties, and in the region about and southwestward of Austin."
I would recommend Buenger's book, among others, for an in depth look at the many factions and sectional rivalries arising in the Lone Star state immediately preceeding and during the Civil War.
Mike