=Crystallography= (_krĭs´tal-lŏg´rȧ-fy̆_).--The science of crystallization, teaching the system of forms among crystals, their structure, and their methods of formation.
=Cyclone= (_sī´klōn_).--A violent storm, often of vast extent, characterized by high winds rotating about a calm center of low atmospheric pressure. This center moves onward, often with a velocity of twenty or thirty miles an hour.
=Denudation= (_dĕn´û-dā´shŭn_ or _dē´nū-_).--The laying bare of rocks by the washing away of the overlying earth, etc.; or the excavation and removal of them by the action of running water.
=Deposit.=--A body of ore distinct from a ledge; pocket of gravel or pay dirt.
=Diplacanthus= (_dip-lä-kăn´thus_).--A fish, belonging to Acanthodii, known only by fossil remains in Old Red Sandstone.
=Drifts.=--Tunnels leading off from the main shaft, or from other tunnels or levels, through and along the vein.
=Drift Matter.=--Earth, pebbles and bowlders that have been drifted by water, and deposited over a country while submerged.
=Druse= (_drṳs_).--A cavity in a rock, having its interior surface studded with crystals and sometimes filled with water.
=Elephas= (_el´e-fas_).--The Latin name for Elephant. The primitive elephant was what is known as the Mammoth.
=Fata Morgana= (_fä´tȧ môr-gä´nȧ_).--A kind of mirage by which distant objects appear inverted, distorted, displaced, or multiplied. It is noticed particularly at the Straits of Messina, between Calabria and Sicily, Italy.
=Fire-damp.=--An explosive carburetted hydrogen of coal mines.
=Fissures.=--Seams or crevices in rocks formed by volcanic or earthquake action, and when filled subsequently by metal or metallic ores they become fissure veins.
=Fog.=--Watery vapor condensed in the lower part of the atmosphere and disturbing its transparency. It differs from cloud only in being near the ground, and from mist in not approaching so nearly to fine rain.
=Geography= (_je-ŏg´rȧ-fy̆_).--The science which treats of the world and its inhabitants; a description of the earth, or a portion of the earth, including its structure, features, products, political divisions, and the people by whom it is inhabited.
ASTRONOMICAL, or MATHEMATICAL GEOGRAPHY treats of the earth as a planet, of its shape, its size, its lines of latitude and longitude, its zones and the phenomena due to the earth’s diurnal and annual motions.