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_ { Recent { Quaternary . . . . { Pleistocene { Cenozoic . . . . { { Pliocene { Tertiary . . . . { Miocene { Eocene { Cretaceous Mesozoic . . . . { Jurassic { Triassic { Permian { Carboniferous . . { Pennsylvanian { { Mississippian Paleozoic . . . . { 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 Sometimes the original hard substance is preserved, but more often it has been replaced by some less soluble material.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 In shells originally made of _aragonite_, a crystalline form of carbonate of lime, that mineral is usually replaced by _calcite_, a more stable form of the same substance.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.