Although originally wirtten decades ago, this is still the definitive work on the subject. His understanding of the sociology and history is so intensely accurate and rigorous that you can see modern Texas in the formative culture of 16th century borderer Scots, the principal ancestors of the Southern U.S. population as well as the Northern Irish Protestant population. Readable and fascinating and not too academic. Especially interesting is his observation that the Scotch-Irish, in their travels from Scotland to the frontier of the USA, tended to stay one step out of reach of the Renaissance and Enlightenment, while remaining devoted sons of the Reformation.