A _natural_ classification may be defined as a concise and systematic statement of the natural relations existing among the objects classified. Now the most important relations existing among rocks are those due to their different origins. We must not forget that lithology is a branch of geology, and that geology is first of all a _dynamical_ science. The most important question that can be asked about any rock is, not What is it made of? but _How_ was it made? What were the general forces or agencies concerned in its formation? Rocks are the material in which the earth’s history is written, and what we want to know first concerning any rock is what it can tell us of the condition of that part of the earth at the time it was made and subsequently. _Classification of Rocks._ +--------------------------------------------------------------------------+ | Sedimentary or Stratified Rocks. | +--------------------------------------------------------------------------+ | MECHANICALLY FORMED. | +-----------------------+-------------------------+------------------------+ | | _Unconsolidated._ | _Consolidated._ | +-----------------------+-------------------------+------------------------+ | _Conglomerate | Gravel. | Conglomerate. | | group._ | | | +-----------------------+-------------------------+------------------------+ | _Arenaceous | Sand. | Sandstone. | | group._ | | | +-----------------------+-------------------------+------------------------+ | _Argillaceous | Clay. | Slate. | | group._ | | | +-----------------------+-------------------------+------------------------+ | CHEMICALLY AND ORGANICALLY FORMED. | +-----------------------+-------------------------+------------------------+ | _Coal | _Iron-ore |_Calcareous | _Metamorphic group (Silicates)._ | | group._ | group._ | group._ | Acidic. Basic. | | | | |/----/\----\ /-------/\-------\ | | | | |85 80 70 60 50 40 30| +-----------+-----------+------------+-------------------------------------+ | Feldspathic. | +-----------+-----------+------------+-------------------------------------+ | Peat. | Limonite. | Limestone. | Gneiss. Diorite. : | | Lignite. | Hematite. | Dolomite. |........................... : | |Bit. Coal. |Magnetite. | Gypsum. | : : : : : |