WALKER, J. Douglas, Department of Geology, University of Kansas, Lawrence, KS 66045, jdwalker@ku.edu, GEISSMAN, J.W., Department of Geosciences, ROC 21, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, BOWRING, Samuel A., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, and BABCOCK, Loren E., School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 S. Oval Mall, Columbus, OH 43210