Where Fossils Occur

Sedimentary rocks, particularly shales and limestones, are the storehouse of 99 percent of the world's fossils. One of the three major types of rocks that form the crust of the earth, these rocks are distinguished from the other two types because they are formed of sediments. Some are composed of silt, sand, and pebbles deposited mechanically by wind and water; some are chemical sediments precipitated from water or taken from water by plants and animals and then deposited with their bodies; and others are sediments that arise from combinations of these agents. Such rocks are commonly stratified or layered, and they are formed of materials and under conditions that are part of our everyday life.

The other two types of rocks — igneous and metamorphic — are the products of more deep-seated dynamic earth forces. Igneous rocks originate as molten magma. They may pour out on the surface as lava and harden, or they may slowly crystallize deep within the earth and then be exposed by erosion. Basalt is an example of volcanic igneous rock, and granite of crystalline igneous rock. Igneous rocks rarely contain fossils; granite never does.

Metamorphic rocks, the third type, are those formed from other rocks by heat and pressure. These agents change the structure of the rock, often recombining its constituent chemicals into new mineral species.

Strange Fossils Found
Volcanic ash may preserve fossils. While excavating solidified ash that covered Pompeii, archaeologists found strange cavities. Plaster forced into them made intricate casts of the bodies of men and animals. In this cast of a dog, details such as the collar are still visible. No bones were found.

Such a working-over is not a favorable situation for the preservation of fossils, although marbles, metamorphosed from fossiliferous limestone, often are cut for ornamental use to display fossil inclusions. Slate (metamorphosed shale) from Bundenbach, Germany, is noted for its spectacular pyritized starfish, trilobites, and other fossils.

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