When the original organic hard parts have been replaced by silica the fossil is said to have undergone silicification, and this type of replacement often produces a very high degree of preservation.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. Replacement by iron compounds Several different iron compounds have been known to replace organic matter. Many Texas limestones contain fossil snails and clams which have had their original shell material replaced by iron compounds such as limonite, hematite, marcasite, or pyrite. Certain of the fossiliferous Tertiary sandstones of the Texas Gulf Coast area contain large amounts of glauconite which commonly replaced organic material. In some areas entire faunas have been replaced by iron compounds. Such is the case in the famous “Pyrite Fossil Zone” of the Pawpaw formation (Lower Cretaceous) in Tarrant County. The fossils in this part of the formation are very small or “dwarfed” and have been replaced by limonite, hematite, or pyrite. Ammonites, clams, snails, and corals are particularly abundant at this locality. TRACES OF ORGANISMS Fossils consist not only of plant and animal remains but of any evidence of their existence. In this type of fossilization there is no direct evidence of the original organism, rather there is some definite indication of the former presence of some ancient plant or animal. Objects of this sort normally furnish considerable information as to the identity or characteristics of the organism responsible for them. Molds and Casts Many shells, bones, leaves, and other forms of organic matter are preserved as molds and casts. If a shell had been pressed down into the ocean bottom before the sediment had hardened into rock, it may have left the impression of the exterior of the shell. This impression is known as a _mold_ (Pl. 2). If at some later time this mold was filled with another material, this produced a _cast_. This cast will show the original external characteristics of the shell. Such objects are called _external molds_ if they show the external features of the hard parts (Pl. 2, fig. 2) and _internal molds_ (Pl. 2, fig. 3) if the nature of the inner parts is shown.