FOSSIL INVERTEBRATES AND PLANTS 99 



with more or less mineralization. Many occur as replacements 

 of the original skeletal substance or hard parts by several minerals, 

 the most important of which is silica (quartz). Mineralizations 

 of wood are found in which the cellular structure is faithfully 

 preserved. Here the cells have been filled and much of the wood 

 replaced by silica, lime, or other minerals. A few animals 

 (graptolites) and many plants are preserved by carbonization. 

 In this process some of the volatile hydrocarbons of the animal 

 or plant have been drawn off leaving only a black film. Through 

 such processes, acting on accumulations of plants, our great coal 

 beds have been produced. 



One of the commonest occurrences of fossils, particularly in 

 sandjr rocks, is in the form of impressions (molds and casts). 

 In such examples the limy, shelly, or skeletal material has been 

 dissolved by percolating water, thus leaving an impression of 

 the exterior of the fossil along with an inner cast representing 

 the mud that filled the interior. Many plants and parts of plants, 

 such as leaves, occur as imprints. 



The following are common fossils: 



Protozoa — usually single-chambered, very tiny shells of infinite 

 variety (fig. 52). 



Sponges — branching, conical, subspherical, or cupshaped, often 

 indefinitely-shaped bodies, having a skeleton composed of fibers 

 or spicules (fig. 53). 



Graptolites — black, lustrous skeletons of colonial polyps usually 

 flattened and suggesting pencil markings on rocks (fig. 54). They 

 are most common in slaty rock. 



Corals — solitary corals are shaped like a goat's horn, while 

 some colonial corals suggest honeycomb (fig. 55). Some more 



