Microelement composition of Gallionella sp.-containing bacterial mats from the environs of St. Petersburg and isotope composition of organic carbon, strontium, and neodymium from these mats have been determined. Isotope and microelement systematics of iron oxides of bacterial origin characterize the geochemistry of aquafacies that contain ferrobacteria. Certain pre-Cambrian ferruginous quartzites have a similar composition; therefore, one may assume that bacterial oxidation of iron under continental conditions had occurred upon the formation of ironstone during the Precambrian.