Each of the dozen basins between the continental slope and the shoreline of southern California is partially filled with sediments. Those nearest shore have the thickest fill, as is indicated by their shallower, wider, and flatter floors and by the coarser median grain size of their fill. Sand layers have been found interbedded in the finer sediments of all except one basin. Cores through the sediments reveal other significant changes with depth. Chief among these is change in water content. A consideration of the water data shows that compaction, owing to weight of overburden, is limited by resistance to grain deformation rather than by low permeability. The interstitial water that is forced upward by compaction provides a means for selective solution at depth and deposition above, just the opposite of supergene enrichment known on land. The generally higher pH and lower organic content at depth may lead to solution of silica at depth and deposition of bedded chert nearer the mud surface. Diagenetic changes of significance in the origin of oil are produced by bacteria. These include changes in both organic and inorganic constituents with depth, and thus with time. The total organic matter and its state of oxidation decrease with depth, with nitrogen disappearing faster than carbon. Sulphate is reduced to sulphide and in some cores totally disappears below about 7 feet. This results in negative oxidation-reduction potentials and formation of pyrite which increases in quantity with depth. These diagenetic processes are influenced by the character of the overlying basin water, which, in turn, is related to the depths of the floor and of the sill of each basin. A continuation of these processes is revealed by a parallel study of the sediments of the now completely filled Los Angeles Basin. The studies lead to an estimate of the efficiency of conversion of planktonic organic matter into oil.