Petroleum and natural gas, wherever found, have probably originated from the decay of organic matter when buried in sedimentary deposits, just as at present in swampy places the hydrogen and carbon of decaying vegetation combine to form marsh gas. The light and heat of these hydrocarbons we may think of, therefore, as a gift to the civilized life of our race from the humble organisms, both animal and vegetable, of the remote past, whose remains were entombed in the sediments of the Ordovician and later geological ages. Petroleum is very widely disseminated throughout the stratified rocks. Certain limestones are visibly greasy with it, and others give off its characteristic fetid odor when struck with a hammer. Many shales are bituminous, and some are so highly charged that small flakes may be lighted like tapers, and several gallons of oil to the ton may be obtained by distillation. But oil and gas are found in paying quantities only when certain conditions meet: 1. A _source_ below, usually a bituminous shale, from whose organic matter they have been derived by slow change. 2. A _reservoir_ above, in which they have gathered. This is either a porous sandstone or a porous or creviced limestone. 3. Oil and gas are lighter than water, and are usually under pressure owing to artesian water. Hence, in order to hold them from escaping to the surface, the reservoir must have the shape of an _anticline_, _dome_, or _lens_. 4. It must also have an _impervious cover_, usually a shale. In these reservoirs gas is under a pressure which is often enormous, reaching in extreme cases as high as a thousand five hundred pounds to the square inch. When tapped it rushes out with a deafening roar, sometimes flinging the heavy drill high in air. In accounting for this pressure we must remember that the gas has been compressed within the pores of the reservoir rock by artesian water, and in some cases also by its own expansive force. It is not uncommon for artesian water to rise in wells after the exhaustion of gas and oil. _Life of the Ordovician_ During the ages of the Ordovician, life made great advances. Types already present branched widely into new genera and species, and new and higher types appeared. Sponges continued from the Cambrian. Graptolites now reached their climax. [Illustration: Fig. 278. Stromatopora] =Stromatopora=--colonies of minute hydrozoans allied to corals--grew in places on the sea floor, secreting stony masses composed of thin, close, concentric layers, connected by vertical rods. The Stromatopora are among the chief limestone builders of the Silurian and Devonian periods.