Page:Popular Science Monthly Volume 50.djvu/399

Rh are found in the different countries of the globe, often associated with salt, gypsum, sulphate of iron, and mineral springs. A considerable number of these deposits are asphaltic or petroleum-bearing basins, of greater or less richness, the working of which requires the boring of wells, or the excavation of galleries permitting a tolerably exact determination of the manner of occurrence of the substances. Nowhere do the works furnish any evidence of the existence of reservoirs or cavities comparable to the caverns of mountainous regions; but everywhere the hydrocarbons are in the state of impregnation or mixture with the rocks in which the workings are made. When they exist in a viscous or solid condition, there is reason for presuming that this state or manner of being is due to particular phenomena of concentration operating at the moment of deposition or after it. The existence of veins or of eruptive beds, ancient or recent, is nowhere established in a certain and indisputable way, but it may be that fissures existing in the rocks have been filled, either from above or laterally, by a posterior displacement.

In attempting to account for the origin of the hydrocarbons, a distinction may be made between two states in which they present themselves, whether on the surface or in the depths of the soil. The initial state, or that of formation, is represented in the stratified beds, where a series of superposed layers is presented. The substance exists, impregnating the rock, and, more rarely, in viscous or solid bituminous masses. The second state, that of alteration or transformation, is met in the beds which have been modified by dislocations, posterior to the solidification of the matters which were deposited in a movable or plastic state. To these dislocations may be attributed the natural petroleum wells which have been known from antiquity, as well as the flows of viscous bitumen which in some regions become solid on exposure to the air.

The slow distillation of marine bodies may be likened, to a certain extent, to the processes of conservation of fossil wood in the bottoms of marshes, peat bogs, etc., under the mud. But while the absence of air is sufficient to assure the preservation of these, the presence of an entirely impermeable envelope is necessary to prevent complete decomposition into volatile gases such as takes place with all animals simply buried under water.

The presence or existence of natural springs of petroleum in the vicinity of mountainous regions is explained, not by dislocations of the ground, but by the fact that the formation of the reliefs is anterior to that of the mineral oil. Instead of regarding these springs as available as guides in researches, they should be regarded as signs of the approaching exhaustion of the arenaceous strata impregnated with oil; these reservoirs not being