petrified		a type of fossil consisting of an extremely thin black film
carbon		an igneous rock layer formed when magma hardens beneath Earth's surface
trace		one of the units of geologic time into which geologist divide eras
period		a fossil formed when an organism buried in sediment dissolves, leaving a hollow area
intrusion		a type of rock that forms when particles from other rocks or the remains of plants and animals are pressed an cemented together
paleontologist		the age of a rock given as the number of years since the rock formed
evolution		the process by which all the different kinds of living things have changed over time
era		the age of rock compared to the ages of rock layers
epoch		a type of fossil that provides evidence of the activities of ancient organisms
relative		a fossil in which minerals replaced all or part of an organism
unconformity		fossil that is a copy of an organism's shape
absolute		an igneous rock layer formed when lava flows onto Earth's surface and hardens
index		a scientist who studies fossils to learn about organisms that lived long ago
fossil		the remains or traces of living things preserved in rock
cast		fossils tell the relative ages of the rock layers in which they occur
mold		a place where an old, eroded rock surface is in contact with a new rock layer
extrusion		describes a type of organism that no longer exists anywhere on Earth
fault		one of the three long units of geologic time between the Precambrian and the present
sedimentary		a break or crack in Earth's lithosphere along which the rocks move
extinct		subdivisions of the periods of the geologic time scale