The paper deals primarily with the application of water analyses to engineering problems in California oil fields. Differences in composition generally are too small or too irregular to suffice as more than rough indications of the horizons from which water enters a well. Electrical devices are more precise. Analyses easily distinguish shallow from deep waters and, in a few fields, top from bottom waters. In some fields the waters of different sands may be distinguished by their total concentration, and in some places by their head.