When the original organic hard parts have been replaced by silica the
fossil is said to have undergone silicification, and this type of
replacement often produces a very high degree of preservation. This is
particularly true of the silicified Permian (geologic time scale, Pl. 1)
fossils from the Glass Mountains in Brewster County. These fossils are
embedded in limestone which must be dissolved in vats of acid, and after
the enclosing rock has been dissolved the residue yields an amazing
variety of perfectly preserved invertebrate fossils (Pl. 3).
Silicified Cretaceous fossils have been recovered from the Edwards
limestone of central Texas. The silicified fauna is restricted to a few
scattered localities, each of which may yield many unusually
well-preserved fossils.