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duration. A subdivision of the periods was needed to refine geological study.
There had to be a smaller-scale unit of time than the periods—a unit recog-
nizable all over the earth. Such a unit was discovered by a French geologist
working in the white chalks of Mesozoic age that are so characteristic of
northern Europe, rocks ultimately named the Cretaceous System.
The great white chalk formations stretching from England across the
length of northern Europe were well known to all of the early geologists, and
surely to the earliest humans in Europe as well, for the chalk yielded the most
important of all Paleolithic tool materials, flint. William Smith himself
knew these formations well, for much of southern England is made of chalk,
and his canals were cut through endless miles of the white rock. Smith rec-
ognized two units in the chalk—one greenish and sandy, the other more rich
in clay. Similar units could be traced beyond England, and in 1822 D'Omal-
ius d'Halloy completed an extensive study of the chalk in France, which he
named
the Terrain Cretace,
or the Cretaceous System. Soon other workers
had added to the understanding of the Cretaceous System.
French geologists discovered that many of the fossils first extracted
from the chalk in England were also present in France. This discovery con-
vinced some naturalists that the same fossils extended everywhere on earth.
A French naturalist named Alcide d'Orbigny became convinced of this and,
using ammonite fossils, subdivided the Cretaceous System into many smaller
units. He named these
stages.
The actual basis for any stage was a fossil ag-
gregate that was unique to it. He was sure that his units allowed the subdivi-
sion of time and that they were recognizable all over the world. Indeed, 150
years later, geologists still use both the term and the concept. The stage
today is recognized as the most refined subdivision of strata—and time—that
is worldwide in extent.
The various stages defined by Alcide d'Orbigny were first recognized in
southwestern France. Fossils defined them, not rock type, yet by chance each
had a slightly different rock type that supported a slightly different type of
wine grape. The stages were named after the regions where they were first
recognized, and some familiar names exist: the Coniacian Stage, named after
