Includes bibliographical references (pages 54-57).
Electronic reproduction. Middleton, Wis. : U.S. Geological Survey, 2000. Mode of access: World Wide Web. System requirements: DJVU plug-in.
Mode of access: Internet from the USGS web site. Address as of 11/18/08: http://pubs.er.usgs.gov/djvu/WRI/wrir%5F99%5F4275.djvu; current access available via PURL.