Pseudomatrix, as defined by Bill Dickinson in 1970, is the term for lithic fragments

Lithic fragment (geology)

Lithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. They were first described and named by Bill Dickinson in 1970. Lithic fragments can be derived from sedimentary, igneous or metamorphic rocks)...

 that have been deformed to become (part of or exclusively) a traditional sandstone matrix

Matrix (geology)

The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...

. This is formed when a lithic-rich sandstone is compacted. The compaction is usually more effective on the (typically) weaker lithic fragments in comparison to the stronger, coarser-grained framework grains.