The whole of geological time has been divided and subdivided according
to varying practices. The development of life is perhaps the one
outstanding feature of the time divisions, but for the most part the
changes in floras and faunas have been gradual rather than abrupt, and
this makes it very difficult to draw sharp lines or to visualize
beginnings and endings of the various stages of development.
Occasionally there is good excuse for drawing a line, where the record
is broken and resumed again after a long lapse of time. The principal
cause of such breaks is the elevation of great land masses, which brings
on an interval of erosion and surface destruction for the areas
uplifted.
These movements of parts of the earth’s crust have been exceptionally
pronounced at certain times, often culminating in the production of
mountain systems, and because of the extreme changes they introduce are
known as revolutions. The major divisions of prehistoric time have been
established, at least in part, by such _revolutions_; crustal, climatic,
or other _disturbances_, on a smaller scale and recurring with greater
frequency, may be regarded as establishing boundaries for the minor
divisions. Hence we have five great Eras of geological history, and
these are divided again into Periods. The time chart shows an
arrangement commonly used in America. In the first column the names of
the Eras are stated in technical form. Closely coinciding with these
terms are the popular names of the Ages which appear in the second
column. These names, describing the dominant life of each age, are very
convenient. The more scientific terms used for the eras, while serving
essentially the same purpose, are a little more systematic and
generalized in that they refer to ancient life (Paleozoic), middle life
(Mesozoic), and recent life (Cenozoic), without being specific as to any
class of animals or plants for any one division of time.
The period names, in the central column, have been derived from
miscellaneous sources, some of them from geographical districts, some
from descriptive references to prominent features of the rocks, others
indicating a degree of approach to recent time. In paleontology (fossil
study) it has long been a practice to cut the periods into lower,
middle, and upper divisions, and in a few cases it has been found
desirable to make two periods out of an old one. What was once known as
the Lower Carboniferous is now commonly recognized as the Mississippian
period while the upper portion has become the Pennsylvanian. The Lower
Cretaceous is now the Comanchean of some authors.
Both old and new practices are responsible for a little confusion at the
present time. A former division into Primary, Secondary, Tertiary, and
Quaternary eras has been partly abandoned, but the term “Quaternary”
still applies to the Age of Man, while “Tertiary time” remains in good
usage for the balance of the Cenozoic era. Among the newer introductions
may be mentioned the use of a Paleocene period which precedes the
Eocene. Geologists are not entirely in agreement as to the necessity for
this addition and many would not give it equivalent rank with other
periods. In the interest of simplicity these modern refinements have
been omitted from the chart.
The figures appearing in the third column, between the Ages and Periods,
indicate the millions of years that have elapsed up to present time.
They denote the age of the rocks at the beginning of each period. The
age of a plant or animal which lived in Eocene time would be, according
to this scale, somewhere between 35 million and 60 million years. In
practice it is usually possible to determine whether a fossil was
embedded in the rocks during an early or late portion of the period, and
thus its age may be established within a shorter range, but it is
impossible to be exact, even in terms of millions of years, with regard
to anything as far back in prehistory as the Eocene period.
The period in which we are living today is known as Recent. It began at
the close of the Ice Age or Pleistocene period about ten thousand years
ago and represents so little of earth history since the beginning of
life that a chart many times the length of this page would be required
to show the rest of the periods in proportion. The Cambrian period is an
early chapter in which the story of prehistoric life suddenly becomes
clear and richly varied. It is, however, much farther from the beginning
of the record than it is from the present, and the Pre-Cambrian eras
would require a great deal more space in order to show their relative
lengths. The Archeozoic and Proterozoic eras have to some extent been
divided into periods, but the great antiquity of the rocks has obscured
much of their history, and divisions established for one locality have
been of little service elsewhere. Consequently, the period names are in
less general use and the common practice is to refer to all this great
stretch of time as Pre-Cambrian.
In the last column, at the right of the chart, some of the historical
features are indicated. This column should be read from bottom to top in
order to get the proper development of the story, and at best this
sketchy outline of events can be no more than suggestive of the progress
and decline through which the earth’s inhabitants have passed.
Rocks of every period except probably the Silurian are known to have
been deposited somewhere in the Colorado area, although in most cases
the record for each period is far from complete. Formations are too
numerous and too varied locally to be shown on a chart of this type.