the sediments of lagoons. The sediments of lagoons with low salinity contain chiefly sandy, clayey, and sometimes clayey-carbonaceous silts enriched by organic matter. In the mineral state the silts are associated with sandy-clayey deposits containing narrow interlayers of marls and limestones and often also interlayers and lenses of coal. In lagoons with unusually high salinity a black silt with a pungent smell of hydrogen sulfide is deposited, composed chiefly of microcrystals of assorted chemogenic formations. Mineral salt-bearing lagoon deposits include strata of sulfate and chlorous sodium salts, gypsum, anhydrite, and, of the carbonaceous rocks, dolomite limestones, dolomites, and sometimes magnesite. Both types of lagoon deposits are characterized by either an absence of fossil fauna or by stunted and specialized forms with rich development of a few species.
Lagoon deposits are distinguished by rapid changes in the rocks and facies both horizontally and vertically, which indicates the transitional conditions (from marine to continental) of their formation. The most frequently encountered mineral products in lagoon deposits are salts, gypsum, anhydrite, and coals. The deposits of lagoons lying in circular coral atolls contain carbonaceous sediments cemented with lime.