74 / Basics of Environmental Science
fossil assemblages can be used to identify particular strata wherever those strata are found. In other
words, animal species have come into existence, lived for a while, and then have disappeared and
their places have been taken by others, newly arrived: we would say evolved. From this it is possible
to construct a ‘stratigraphic column’, a vertical section through sedimentary rocks in which each
stratum is shown in chronological order. The Cuvier and Brogniart study inspired geologists all over
Europe to apply the method to their own localities and eventually to divide geologic time into distinct
episodes on the basis of the animals associated with them. Though much amended, the geologic
time-scale used today is derived from this work, as are most of the names.
Geologic time is divided into eons, eras, suberas, periods, and epochs. Table 2.4 shows the present
arrangement, starting with the oldest, although some of the dates are revised from time to time (Ma
means millions of years ago). Priscoan, Archaean, and Proterozoic together comprise the eon formerly
known as the Precambrian. The term ‘Precambrian’ is still widely used, but not in a formal sense (all
it means, after all, is ‘before the Cambrian’).
The order in which historical episodes should be arranged having been established, the next
step is to allot dates to them. The thickness of strata is no help with this. Sedimentation is an
Table 2.4 Geologic time-scale