Kerogen was believed to be formed by the inspissation of petroleum. During this process nitrogen and sulfur compounds were concentrated in the most inspissated or weathered products. At a certain stage, reached gradually, the organic matter became insoluble in carbon-disulfide and ceased to be a bitumen. Oil shale was formed by the power of certain clays or shales to absorb inspissated petroleum, particularly unsaturated hydrocarbons. This adsorption apparently depended on the colloid content of the argillaceous rock. This rock retained these impregnated petroleum residues long after porous sandstones in the vicinity had lost all traces of petroleum by weathering and leaching.