Oil shales and coals originated in fresh water muds that contained large amounts of spores, algae, and other nonwoody vegetable material. This organic debris was partly decomposed by bacterial action but not enough to increase the percentage of fats by removal of other plant substances. By contrast, petroleum was formed by thorough decomposition of nonfatty material in salt water. The main difference in bacterial action was due to differences in the saline content of the water in which the organic material was deposited. In fresh water, the amount of decay was small, whereas in salt water it was nearly complete.