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STUDY QUESTIONS |
One apparently obvious way to correlate (tie together) these local sequences
might be to trace the layers exposed in
First of all, rock layers do not stay constant in character over long
distances. Rocks are a reflection of their
Secondly, the same rock layer will only rarely be of the same age in
different places. If, for instance, we were to find a deposit of beach
sand covering several thousand square miles, it would stand to reason that
there was no beach of that
To correlate these sequences we need distinctive unique time markers which will enable us not only to tie things of similar ages together, but also to compare and arrange sequences which are not in the same area in chronological order. Occasionally there are events which are distinct, unique, and affect such large areas as to provide us with an almost instantaneous time marker. An example might be a bed of volcanic ash that settled over a large area as a result of a volcanic explosion. Such marker beds, as they are called, do exist, and are extremely useful. Unfortunately, they are rare, random, and non-universal events and therefore have only limited application in a history which spans nearly five billion years. Only ONE record in all of geologic time has been continuous and ever changing while being common enough to have left traces over virtually the entire earth and that is the record of life, as represented by fossils.
FOSSILS
In the late eighteenth century, two people, an English engineer named William Smith and a French paleontologist, Baron George de Cuvier, discovered independently how useful the fossil sequence is for correlation, even though its cause (evolution) had not yet been figured out. By the mid-nineteenth century, fossils had become universally accepted as tools for correlation, not only on a local, but also on a world-wide basis. For fossils to be of use for correlation, it is implicit that no two species can be identical either at a given moment in time, or over time. The discovery of millions of species in the fossil record has amply confirmed these assumptions. As can be expected, some species are more useful tools for correlation than others. We call such useful species "index fossils" because they act as time markers. To be an index fossil, the species must be common, widespread geographically, easily recognized, and rapidly evolving. i.e., short-lived as a species from a time standpoint. Actual correlation between two local sequences, and dating in general,
are done by comparing the stage of evolution of the fossils contained in
a local sequence to a standard evolutionary sequence which has been worked
out on a
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