It would be a serious mistake to regard nature as divided into a number
of distinct and independent schools of fossil making, each refusing to
use the methods and devices of another. There are, however, certain
features which stand out so prominently that a little classification
becomes helpful. While this procedure brings out differences it should
be understood that processes actually work together, several of them
usually being involved in the production of any individual specimen.
(1) Impressions of animals and plants, or parts of these, are frequently
left in soft sand or mud which later becomes converted into more durable
rock. This type of fossils is represented by animal foot-prints and the
imprints of leaves, flowers, insects, and like objects which may be
mingled with the finely ground materials of the common sedimentary
rocks.
(2) Parts of plants and animals may be gradually replaced by mineral
matter with little or no change from original form and texture. Fossils
of this class are said to be petrified or turned to stone. They are also
known as replacements. The fleshy parts of animals do not petrify.
(3) Many animals among the invertebrates use mineral substances for
protective or supporting structures. Small plants of various kinds
follow a similar practice. These structures, being produced in stony
materials, are readily converted into fossils. The shells of mollusks
are the best known illustrations in this field, and all that is required
for a shell to become a fossil is the extinction of the species of
animal that produced it. Fossils of this type are extremely abundant.
(4) Preservative substances other than those which produce common rocks
may be mentioned among fossil-making possibilities. Bones are known to
have been preserved in asphalt, and insects in resins, but such cases
are few in comparison with the products of other methods.
(5) In rare instances there has been preservation of extinct creatures
by the process of drying or by refrigeration. Occasional mummies are
found with shriveled flesh and skin still in place, but better
preservation of all tissues occurs when the temperature is quickly
reduced below freezing point and held there without interruption. This
can happen only in the colder parts of the earth and is always subject
to climatic change. The effect of drying also may be undone at any time
by a slight increase in the amount of moisture.
(6) Coal beds often produce fossils of an unusual sort. In the formation
of coal, plant material gradually loses some of its more perishable
substances but retains carbon which has better lasting qualities and
slowly accumulates to produce the seams and beds that are mined. In the
early stages of the process the original vegetation undergoes little
change in appearance but eventually practically all of its character is
lost. Many fossil leaves are found as thin layers of carbon, bedded in
the clays which are commonly associated with coal deposits.
(7) Concretions, which are hardened lumps of mineral substances
occurring commonly in sandstones and shales, are often mistaken for
fossils because of their peculiar shapes. However, there are localities
in which the mineral solutions have been concentrated and deposited
around shells, leaves, seeds, or similar objects, thus producing an
abundance of fossils which may be obtained by opening the concretions.
Fossils of this type are well known from Mazon Creek and other districts
in Illinois, Kansas, Colorado, and elsewhere.