When such petroleum is heated, the lighter oils in it, such as gasoline and kerosine, are driven off, leaving the heavy, sticky asphalt. Native asphalt is probably also a residue of petroleum, left when the lighter oils evaporated or seeped into rock fissures. It is more solid than petroleum asphalt and is sometimes brittle. When heated and fluxed with petroleum oils, it softens and becomes sticky. Petroleum asphalt does not require fluxing.
Native asphalt occurs widely.
In some places the deposits appear on the surface of the Earth and are so large they are called lakes. Lake asphalt is semisolid and is dug out in chunks. It is identical in its properties to impure petroleum asphalt. Another form of native asphalt, called vein asphalt or asphaltic coal, is mined from rock crevices. Jet black, brittle, and lustrous, it is the purest of all asphalts.