Nicholas Clayton's masterpiece
Nicholas Clayton's religious masterpiece was the Ursuline Academy in Galveston, which unfortunately was torn down in the 1960's due to damage by Hurricane Carla. In the 1900 storm there are estimates that as many at 1,000 persons had their lives saved by retreating to the convent. While it's gone, you can at least get a sense of it from this picture.
If you are interested in some of his non-religious works, this site has some nice pictures.