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Containing magnesium and iron and only a very small amount of silica. Used of igneous rock.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Geological Science) (of such igneous rocks as peridotite) containing less than 45 per cent silica
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
ul•tra•ba•sic(ˌʌl trəˈbeɪ sɪk)
(of rocks) containing iron and magnesium, with little or no silica.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.