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The indications of Purbeck deposits in this neighbourhood, although the beds are neither very extensive nor of any great thickness, are more clearly defined than has been generally supposed. There area bout thirteen feet of strata decidedly freshwater, containing Cypris, Paludina, Planorbis, and remains of vegetables, chiefly Thuytes and small seed-vessels. The western end of the great quarry at Swindon presents the following section in descending order: There can be no doubt therefore that these beds are representatives on a small scale of a portion of the Purbeck formation. The surface of the Portland strata has been greatly denuded previously to the deposition of the overlying group, for in many cases the latter is deposited in hollows and cavities where the Portland sand has suffered erosion by water, on which it reposes at very irregular intervals. The top of the Portland oolite is composed chiefly of whitish sand containing layers of Trigonia and other marine shells, and this includes harder nodules full of shells, chiefly Trigonia incurva , Lucina Portlandica , Cytherea rugosa and Nerita angulata , often retaining their shelly covering though decomposed and friable. These masses too are very unevenly dispersed. No. 5 in the section, which in its lower part is highly carbonaceous and resembles vegetable mould, is to al appearance a kind of ‘dirt bed,’ and may be considered as the representative of one the dirt beds in the islands of Portland and Purbeck. It also contains numerous small pebbles which are likewise found in it on the coast