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Branchlike aggregates, several millimeters in length, of microscopic pyrite spherules are abundant in a narrow zone in upper Ancylus clay deposited approximately 7 500 yrs B.P. in the southeastern part of the Bothnian Sea. The aggregates in general, and often also the individual pyrite spherules, have crusts of marcasite exhibiting a radial growth pattern.In connection with stratigraphic work on Quaternary clays from the marine area west of the town of Pori, SW-Finland, small pyrite concretions were found to abound in a rather limited zone in gray homogenous clay. Samples containing concretions were taken with a modified Kullenberg piston corer. The cores, taken as far as 25 miles apart, all exhibited exactly the same sedimentary sequences.The Quaternary clays ( Fig. 1) have been deposited on till of the last glaciation. The lowermost sediments are varved clays and silts whose varve thickness decreases upwards exponentially (Ignatius, 1958). They grade into a more or less black sulphidic clay, which in turn changes rather abruptly into the gray homogenous clay containing the pyrite concretions. A few centimeters above the pyrite zone the gray clay as a rule changes sharply into a greenish gyttja-banded clay, which is overlain by a recent, dark, greenish, sulphide bearing, homogenous clay.The pyrite concretions are mainly concentrated in a layer 1-2 cm thick. No pyrite was detected in the few centimeters of gray clay separating the gyttja-banded clay from the sharp upper limit of the pyritic zone. Downwards the amount of pyritic concretions decreases gradually to vanish completely only slightly above the sulphidic clay.According to pollen and diatom analyses on cores from the Bothnian Sea the gyttja-banded clay was deposited during the marine Littorina stage of the Baltic Sea evolution (Ignatius, 1964) and corresponds to pollen zones VI-VIII. The gray clay below the gyttja-banded clay corresponds to the fresh water Ancylus stage and the upper part of zone V. Zone boundary V/VI, however, lies somewhat below the boundary between the gyttja-banded clay and the gray homogenous clay thus coinciding rather well with the main pyritic zone. The black sulphidic clay was deposited in the early part of the Ancylus stage corresponding to pollen zone IV.The age of zone-boundary V/VI and thus also the age of the pyritic layer has been estimated to be 7 500 yrs. (Kukkonen, Manuscript under preparation).The pyrite concretions exhibit variations both in size and shape (Fig. 2). The largest individuals were almost a centimeter in length but at the