As a result of the writer’s field work on the Green River formation, and his study of the microfossils of the oil shale and of the analogy of the shale with the deposits of modern lakes, there appears to be the following trend in the formation’s geologic history: The Green River lakes were formed as relatively stable, shallow, fresh-water bodies; under climatic influence they gradually changed to lakes that periodically filled and evaporated but that supported a luxuriant microflora. In the third stage they became quite strongly alkaline ponds, perhaps playa-like, in the muds of which considerable glauberite crystallized out. In the shallow, expanding and contracting fresh or mildly alkaline lakes of the intermediate stage organic ooze consisting almost wholly of plankton organisms (dominantly microscopic algae) accumulated in large quantities. Bacteria caused active putrefaction of the ooze and evaporation reduced the decaying mass to a syrupy consistency or occasionally to actual dryness. This macerated organic matter was covered by the deposits of the next cycle and subsequently lit hi lied into oil shale.