For example, in 1902 there were collected the first fossils ever found on the antarctic continent. Among the dozen specimens obtained were some fossil ammonites (a family of chambered shells) of genera which are found on other continents in certain formations classified as the Cretaceous system, and which occur neither above these formations nor below them. On the basis of these few fossils we may be confident that the strata in which they were found in the antarctic region were laid in the same period of geologic time as were the Cretaceous rocks of the United States and Canada.
The record as a time scale. By means of the law of included organisms and the law of superposition the formations of different countries and continents are correlated and arranged in their natural order. When the geological record is thus obtained it may be used as a universal time scale for geological history. Geological time is separated into divisions corresponding to the times during which the successive formations were laid. The largest assemblages of formations are known as groups, while the corresponding divisions of time are known as eras. Groups are subdivided into systems, and systems into series. Series are divided into stages and substages,—subdivisions which do not concern us in this brief treatise. The corresponding divisions of time are given in the following table.
| Strata | Time | |
| Group | Era | |
| System | Period | |
| Series | Epoch |
The geologist is now prepared to read the physical history—the geographical development—of any country or of any continent by means of its formations, when he has given each formation its true place in the geological record as a time scale.
The following chart exhibits the main divisions of the record, the name given to each being given also to the corresponding time division. Thus we speak of the Cambrian system, meaning a certain succession of formations which are classified together because of broad resemblances in their included organisms; and of the Cambrian period, meaning the time during which these rocks were deposited.
| Group and Era | System and Period | Series and Epoch | ||
| Cenozoic |  | Quaternary |  | Recent Pleistocene |
| Tertiary | | Pliocene Miocene Eocene | ||
| Mesozoic | | Cretaceous Jurassic Triassic | ||
| Paleozoic | | Carboniferous | | Permian Pennsylvanian Mississippian |
| Devonian Silurian Ordovician Cambrian | ||||
| Algonkian | ||||
| Archean | ||||
Fossils and what they teach
The geological formations contain a record still more important than that of the geographical development of the continents; the fossils imbedded in the rocks of each formation tell of the kinds of animals and plants which inhabited the earth at that time, and from these fossils we are therefore able to construct the history of life upon the earth.
Fossils. These remains of organisms are found in the strata in all degrees of perfection, from trails and tracks and fragmentary impressions, to perfectly preserved shells, wood, bones, and complete skeletons. As a rule, it is only the hard parts of animals and plants which have left any traces in the rocks. Sometimes the original hard substance is preserved, but more often it has been replaced by some less soluble material. Petrifaction, as this process of slow replacement is called, is often carried on in the most exquisite detail. When wood, for example, is undergoing petrifaction, the woody tissue may be replaced, particle by particle, by silica in solution through the action of underground waters, even the microscopic structures of the wood being perfectly reproduced. In shells originally made of aragonite, a crystalline form of carbonate of lime, that mineral is usually replaced by calcite, a more stable form of the same substance. The most common petrifying materials are calcite, silica, and pyrite ([p. 13]).