1. Department of Mathematical and Statistical Sciences, Marquette University, Milwaukee, WI 53201-1881 USA.
2. Department of Statistical Science, Duke University, Durham, NC 27708-0251 USA.
3. The Lyell Centre, British Geological Survey, Edinburgh, EH14 4AP, UK, and The School of Geosciences, University of Edinburgh, Edinburgh, UK.
4. Department of Earth, Marine, and Environmental Sciences, University of North Carolina, Chapel Hill, Morehead City, NC 28557 USA.