The minerals occurring in each division of a Pennsylvanian cyclothem at St. Louis, Missouri, have been determined and have yielded information about their sources and conditions of deposition. The climate was warm and moist both in the area of deposition and in the area, Ozarkia, from which the sediments were derived. At the beginning of the Pennsylvanian, the land was elevated as a whole and laterization took place forming a regolith composed of chert and sometimes sand at the top, ranging down through halloysite and kaolinite clay, through kaolinite clay and chert to illite clay and chert and sand, or to limestones containing them. The land rose in the upland regions giving rise to overloaded streams and the formation of alluvial plains and lagoons in which the materials of the above laterized regolith were laid down, topmost formation first. The land was sinking slowly and about the time the illite was reached, swamp conditions had come into being with the accumulation of decaying vegetation from which coal was to form. Subsidence quickened and the sea invaded the continent. However, the sea at first was shallow and conditions were good for the formation of pyrite. Meanwhile the illite, or rock containing illite, was being eroded and as the sea became deeper, conditions for the formation of pyrite became less favorable and shales containing illite and limestones were formed. The land began to rise and finally the sea left the land. Laterization again took place and the process was repeated.