PURBECK BEDS, a group of strata forming the upper members of the oolitic period (q.v.), and so named because they are well developed in the peninsula called the isle of urbeek (q.v.), s. of Poole estuary in Dorsetshire. They are, like the Wealden beds above them, chiefly fresh-water formations; but their organic remains join them more closely to the marine-formed oolites below than to the superior Wealden series. Though of a very limited geographical extent, the Purbeck beds have yet considerable impor tance from the changes in animal life that took place during their deposition. Generally less than 200 ft. in thickness, they, however, exhibit three distinct and peculiar sets of animal remains. This has caused them to be arranged into three corresponding groups, known as the upper, middle, and lower Purbecks.
The upper Purbecks are entirely fresh-water, and the strata are largely charged with the remains of shells and fish; the cases of the entomostraca cyprides are very abun dant and characteristic. The building-stone called Purbeck marble belongs to this division.
The middle Purbecks record numerous changes during their deposition. The new est of the strata consists of fresh-water limestone, wills the remains of cyprides, turtles, and fish. This rests on brackish water-heds—cyrena with layers of corbula and melanin.
Below this there are marine strata, many species of sea-shells. Then follow some fresh and brackish-water limestone aneshales, which again rest on the cinder-bed, a marine argillaceous deposit, containing a vast accumulation of the shells of a small oyster. This is preceded by fresh-water strata, abounding in the remains of entomos traca, and containing some beds of cherty limestone, in which little bodies, believed to have been the spore-cases of species of chart, have been found. At the base of this sub grOup, a marine shale occurs, containing shells and impressions apparently of a large zostera.
The lower Purbecks begin with a series of fresh-water marls, containing entomos, traca and shells. These rest on strata of brackish-water origin; and then follows a sin gular old vegetable soil, containing the roots and stools of cycads, and the stems of coniferous trees. Front its black color and incoherent condition, this layer has received from the quarrymen the name of the " dirt-bed" (q.v.). This rests on the basement-bed of the whole group, which is a fresh-water limestone, charged with entomostraca and shells, and contains the thin layer in which Mr. Heckles has lately found the remains of several species of mammalia. '