Given enough time, the remains of an organism are likely t be modified.Orten, fossils become petrified(literally, tumed into stone"), meaning that the small internal cavities and pores ofthe original structure are filled with precipitated mineral matter(Figure 1 In other instances replacement may occur. Here the cell walls and other solid material are removed and replaced with mineral matter sometimes the microscopic details of the replaced structure are faithfully retained of fossils. and casts constitute another common class When a shell or other structure is buried in sediment and then dissolved by underground water, a mold is created. The mold faithfully reflects only the shape and surface marking of the organ ism; it does not reveal any information concerning its internal structure. these hollow spaces are subsequently filled with min eral matter, casts are created(Figure l1.13B) A type of fossilization called carbonization is particularly effective in preserving leaves an delicate animal forms. It occurs when fine sediment encases the remains of an organism. As time passes, pressure squeezes out the liquid and gaseous components and leaves behind a thin residue of carbon(Figure 11.13C) Black shales deposited as organic-rich mud in oxygen-poor environ ts often contain abundant carbonized remains. If the of carbon is lost from a fossil preserved in fine-grained sediment, a