Fossils are the remains or traces of former living creatures
or plants now preserved in the rocks. Some kinds of rock,
e.g. limestone, consist entirely of the remains of plants
and animals in various stages of decay and decomposition;
but nearly every kind of sedimentary rock may be more or less
fossiliferous. The parts whose form is preserved may be the
bones, teeth or shells of animals (chiefly marine); the bark,
wood, leaves or seeds of plants; and even the foot prints
or tracks of animals. Sometimes, in the more recent strata,
the actual hard parts of an organism are preserved unaltered,
but usually only an “impression,” or a “cast,”
or a “petrifaction” remains.
An impression is the manner in which the foliage of plants
is often preserved. In shows the external shape and ornamentation
of the plant, generally as a thin, black, carbon thin film.
Impressions are common in the shales above coal seams.
Casts. These may be either external, internal or hollow.
external cast is formed when an organism has been covered
with sediment and then disappears by decomposition, leaving
a hollow cavity or mould showing the shape of the outside
of the body. This mould has later become filled with mineral
matter, producing an external cast which shows all the external
markings of the body but not its internal structure.
An Internal cast shows only the interior markings of a hollow
structure (e.g. an empty shell) being formed when sediment
fills the vacuity and the enclosing organism subsequently
disappears, leaving a stony kernel.
A hollow cast is a combination of (a) and (b) whereby the
hollow space or annulus formed between the outer mould and
the internal cast by the decomposition of the organism becomes
filled with other material. The latter then shows both external
and internal markings.
LIFE OF TRIASSIC TIME
Land Plants. The plants of this time are still imperfectly
known, for less than 400 species have been described from
all the world, and these are chiefly from the Upper Triassic
formations. This situation may be due to an actual impoverishment
of plant life because of the harsh climates, but it more probably
results from the fact that redbeds are a poor environment
for preservation of plants.
The Petrified Forest has yielded chiefly petrified logs, although
foliage has been found in several places,3 re-cording eyeadcoids
and ferns that grew along the stream courses. The logs are
of conifers, not unlike the great pines. Many of the logs
are of noble size, some attaining a diameter of 10 feet at
the base and a length exceeding 100 feet. It has been estimated
that some of these trees stood nearly 200 feet high. They
now lie im-bedded in the Chinle shale; petrified as agate.