Man's physical universe

xanabras

170 THK KARTH AS MAN'S ABODE

fossils. Otherwise fungi, bacteria, or animals soon destroy them. Only

a few fossils have been formed from soft-bodied organisms.

Carcasses of woolly mammoths have been found frozen in Siberia,

so well preserved that their flesh was still edible after thousands of

years. Fossil remains consist of the shells, skeletons, and other hard

parts of living organisms which contain enough mineral matter to

enable them to resist decomposition long enough for them to be covered

by deposits which have preserved them in stratified rock until the

present time.

Fig. 53. Uncovering Brontosaurus bonet, l; hl Cal)in (Juarry, Wyoming.

(Courtesy of the American Museum of Natural History.)

Often pseudomorphs are found in the older rocks. A pseudomorph is

a fossil which has been preserved by the substitution of the original

material by mineral matter in such a way as to preserve the original

shape and features of the specimen. Petrified trees are pseudomorphs.

Frequently fossil molds or casts are found. A mold is a cavity left in a

rock by the complete decay or solution of a dead organism, leaving its

form in the sediment. If a mold is later filled with mineral matter

which retains the shape of the mold, the fossil is called a cast.

Tracks, trails, and furrows have also been preserved in the rocks.

Peat bogs and asphalt pits have proven to be especially good sources

of fossils of animals which have been trapped in them.

Coal deposits

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