Page 96 - The Restless Earth Fossils

glossary 5

phanerozoic eon The period of time from 542 million years
ago to the present.
precambrian Geological time from the origin of the Earth
until 542 million years ago.
principle of original horizontality Sediments were level
when first deposited.
principle of superposition Older sediments lie beneath
younger ones.
proterozoic eon Period of time from 2.5 billion years ago to
542 million years ago.
pseudomorph A fossil cast having only the shape and outer
impression of a tree or other organism.
pterosaurs Extinct flying reptiles of the Mesozoic Era.
quartz White, often translucent mineral made of silicon
dioxide.
radioactivity Emission of X-rays by an element.
relative time Age of a fossil or formation as compared to
another fossil or formation.
replacement When inorganic minerals replace organic com-
pounds during fossilization.
resin Tree sap.
rock matrix The rock surrounding a fossil.
sandstone Stone formed from compressed and heated sand.
sauropod Long-necked vegetarian dinosaur.
scanning electron microscope Microscope using electrons
rather than light to form an image of small objects.
search image Mental image of what to look for in the field.
sedimentary rock Rock formed from the heating and com-
pression of sediments originally laid down in water.
shale Sedimentary rock with a high clay content.
shocked quartz Quartz modified by the extreme heat and
pressure of a large meteorite or asteroid.
silt A deposit of mud or fine soil at the bottom of standing
water.

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