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laboratory instruments; thousands are operated daily throughout the world.
Some investigators continue to look at the relative isotopic composition of
earth material to produce reliable dates. Others examine lighter isotopes of
carbon and oxygen in order to measure ancient temperatures. Many scien-
tists, however, are still dedicated to finding the age of rocks and to solving
the most interesting question of all: How old is the earth? The most recent
measurements suggest an age of at least 4.5 billion years.
Returning to rocks
Ironically, the techniques of radiometric age dating work best on those rocks
that most stubbornly resisted nineteenth-century efforts to determine ages—
the igneous rocks, which never contain fossils. Radiometric dating methods
work least well on those rocks that contain fossils. The most sophisticated
mass spectroscope in the world is useless on an ordinary fossil-bearing sedi-
mentary rock. If a sedimentary rock is to be dated radiometrically, there must
be some type of interbedded lava flow or ash layer locked within. Unfortu-
nately, such events were relatively rare.
Just as clearly, the majority of sedimentary rocks cannot be directly
dated. The only solution was to find a way to date rocks in those places
where sedimentary rock units and volcanic rock units intersected in time—
places where ashes or lava flows can be found interbedded with ancient sed-
imentary rocks. Such places became the new holy grail that geologists sought.
Paleontologists and other geologists learned a new word and learned to rec-
ognize a new rock type: bentonite, a thin orange layer of lithified volcanic
ash found in sedimentary rock.
Arriving at age dates for Cretaceous sedimentary rock has been a par-
ticularly vexing problem. The original type areas exposed in Europe were de-
posited in tegions far from active volcanoes so they have few or no ben-
tonites. Much better beds are found in the western interior of North
America, east of the Rocky Mountains—the site of the ancient Western In-
terior Seaway.
