Definition of "groundmass" []

• The fine-grained crystalline base of porphyritic rock in which larger crystals are embedded. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The rock here is a porphyritic granite (porhyritic meaning that the stone has large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass)."
• "[Small, separated mineral crystals in a glassy groundmass]"
• "Immersed in this groundmass are a wide variety of rock shapes, sizes, textures—round and angular—many with varying consistencies."