Also known as siliceous sinter. The lighweight, porous, opaline variety of silica that is white or nearly white and deposited as an incrustation by precipitation from the waters of geysers and hot springs
U.S. Geological Survey, U.S. Department of the Interior, Volcano Hazards Program, Glossary, Washington, DC, USA 2011. http://volcanoes.usgs.gov/vsc/glossary/alluvium.ht ml, (ND/ Volc Emr - Amer) {795}