MIDDLETON, Matthew Corbett, Department of Earth Sciences, St Francis Xavier Univ, Augustine Hall, P.O. Box 5000 St.FX University, Antigonish, NS B2G 2W5, Canada, KEPPIE, J.D., Departamento de Geologia Regional, Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universiaria, Mexico, MURPHY, J.B., Department of Earth Sciences, St Francis Xavier Univ, Augustine Hall, P.O. Box 5000, Antigonish, B2G 2W5 and NANCE, R.D., Department of geological Science, Ohio Univ, Ohio-Athens, OH, mmiddlet@stfx.ca, mmiddlet@stfx.ca