Definitions

1. noun

the coarse-grained, solid part of an igneous rock, consisting of the accumulated intergrowths of small crystals of various minerals.

“The geologist examined the groundmass of the volcanic rock to determine its composition.”

2. noun

the solid part of the earth’s crust beneath the surface, as opposed to the soil or topsoil.

“The groundmass beneath the city made it difficult to construct new buildings.”