Footnote:
Mode of access: Internet from the USGS web site. Address as of 10/22/07: http://pubs.usgs.gov/of/2007/1005/pdf/OFR2007-1005.pdf; current access via PURL
Mode of access: Internet from the USGS web site. Address as of 10/22/07: http://pubs.usgs.gov/of/2007/1005/pdf/OFR2007-1005.pdf; current access via PURL