Man's physical universe

xanabras

THE HISTORY OF THE EARTH 171

often show patterns of leaves, while volcanic ash and lava yield fossil

remains that have been caught and petrified in these deposits. In the

Mojave Desert there is a thick stratum of volcanic ash that probably

settled in a lake and covered over branches and trunks of trees, which

were then petrified as they lay in this sediment at the bottom of the

lake.

The fact that the types of fossils in different rock layers vary from

simple one-celled organisms in the lowest rock layers to the complex

mammals in the top layers indicates that life has not always existed

Fig. 54. Dinosaur tracks in sandstone in the Peace River Canyon, British

Columbia. (Courtesy of the Department of Mines and Resources, Canada.)

on the earth in its present forms but has passed through an evolutionary

process. Repeated observations have shown that rocks of a given

period contain fossils peculiar to that period.

Geologists who specialize

in the study of fossils are called paleontologists ; they are able to date

a given stratum by an examination of the microscopic and other fossils

which it contains. This identification of strata is very important in

petroleum geology, because it enables the geologist to examine oilwell

cores and tell what stratum is being drilled and when a stratum

which usually contains oil may be expected to be reached.

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