used for fuel at that place on the coast of the Isle of Purbeck. These beds are well seen on the coast of the Isle of Portland. The slate clay contains both animal and vegetable impressions. After being heated, the shale divides into large tabular masses. These beds furnish the selenites of Oxfordshire, which are constantly in progress of forming, by the union of the acid of oxygenised pyrites, with the calcareous matter of the oyster shells, and other marine fossils. The most interesting organic remains of this stratum are those of the extinct genera of the fishlacertæ. The vertebræ, paddle-fins, &c. of a species of ichthyosaurus, different from the ichthyosauri of the lias, have been observed; as also, the vertebræ, phalanges, and head, probably of a plesiosaurus. Bones like those of the cetacea, also occur. Of the shells present in abundance, the ostrea deltoidea seems most characteristic. The genera are nearly as in the coral rag. See Plate IV. 2. PORTLAND OOLITE.-Several beds of a coarse earthy limestone, compose this formation. Through all the three divisions of the oolite series, the limerocks are so similar, as to be distinguishable chiefly by their organic remains. All their varieties possess the character of a yellowish white calcareous freestone, mixed with a small quantity of siliceous sand, and becoming oolitic ; features in which they differ entirely from the argillaceous beds. The upper series, called the oolite of Aylesbury and Portland, is fine grained, white, loose granular, of earthy aspect, with various shades of yellowish gray. Occasionally it occurs as a compact cretaceous SECTION OF THE PORTLAND QUARRIES. 269 limestone, with a conchoidal fracture. In Wiltshire and Dorsetshire layers of chert alternate with the limestone, like flints in the chalk; while the lower beds have a sandy aspect from numerous particles of green sand. Those varieties which are most distinctly oolitic, as at Purbeck and Portland, afford a fine building stone, much employed in London. In the Isle of Portland, the uppermost beds are numerous. They consist of oolitic rock, which at the very summit, called the cap, is yellow and porcellanous; it is fit only for making lime. The next bed is quarried for the builders; the beds below, being shelly or cherty, are employed for coarser purposes. The following section of the quarries on the west side of Portland, was published in the Monthly Magazine for 1813, p. 481. 2, and 3. In Gosling's quarry, the stone brash and two beds of the cap are together, 20 4. Roach, in one bed 4 feet, and 2 other feet are united to the top of the white bed, in all 6 5. White bed, a marketable building stone, exclusive of the two feet of Roach united to it, 8 Many beds of flint and stony rubbish, 6. Two other beds of Roach in the place of the middle bed of saleable stone, 7. The third bed of marketable stone, N.B. On the side of the island this bed is of better 8. The above series lies on many layers of flints and beds of The whole of the rocky strata, 96 66 55 or 60 112 Beneath is black blue shiver several hundred feet thick, of which about 100 feet stand above the level of the sea. The beds at Portland and Tisbury contain beautiful yellow sulphate of barytes, called sugar candy stone, and calcareous spar. The organic remains are chiefly shells. The Amonites triplicatus and Pecten lamellosus are characteristic. See Plate IV. and Geol. of Engl. p. 176. Large fragments of wood are common, and beautiful silicified plants (Cycadeoidea). See Geol. Trans. June 1828. The soil of this stratum is a poor stone brash. The water intercepted by the subjacent Kimmeridge clay is thrown out copiously from the bottom of the oolite. 3. PURBECK BEDS. These which form the summit of the series, are composed of many thin layers of argillaceous limestone, which alternate with slaty marls, and form altogether a body 300 feet thick. According to Mr. Webster, the Purbeck stone consists chiefly of shells (principally the Helix Vivipara), partly entire and partly comminuted, imbedded in a calcareous cement, which is sometimes pure and crystalline, and at others like an indurated marl. Between these beds are interposed others free from shells, along with layers of marl and shale, which, by its shivery nature, facilitates the quarrying operations. The hill contains many alternations of these strata. One good freestone bed, worked and shipped off for London in Purbeck squares, is 5 feet thick ; another a little below it, called the new vein, is of the same thickness. The uppermost useful stone is called the leaning vein. It is 7 feet thick, and affords flagstones for pavement. The only mineral contents of these beds are the pyrites of the marl stratum, and the sulphate of lime, produced by its action on the calcareous matter. Beautiful impressions of fish are frequently found STRATA OF THE ISLE OF PURBECK. 271 by the quarrymen between the lamina of the limestone, as also fragments of bones, belonging chiefly to the turtle. Complete fossil turtles have also occurred, of which one specimen was found in great perfection. The shells of this formation have not been accurately examined. The Purbeck beds have an average total thickness of about 300 feet. They are retentive, and must be sunk through for water. None of the above formations of the upper division of the oolite series, have been noticed hitherto to the north of Buckinghamshire. Here the Portland beds first make their appearance beneath the iron sand, forming a constituent of the same chain of hills. It underlies a great part of the vale of Aylesbury. On the borders of Oxon and Bucks, it rises from Thame, and culminates on the insulated group of Brill hills, with a thin covering of iron sand. It passes thence to the south-west, and again culminates at Shotover. Through the north of Dorsetshire, the prolongation of this series is concealed by the projection westwards of the vast overlying platforms of chalk and green sand extending over their basset edges or outcrops. Beyond the escarpment of the chalk it reappears. The strata of the isle of Purbeck district are inclined to the horizon, and are perforated with caves at its western extremity. There the Portland beds, dipping inland in an angle of from 45 to 60 degrees, form the exterior barriers and capes at the mouth of these coves, while the vertical strata of the high chalk downs range along their bottom. The more solid masses of the Portland rock having most powerfully resisted the destroying agents which seem to have excavated the coves, often form a reef of rocks in advance. In the convulsions of the strata the solid beds of Portland rock have been lifted up in mass, in regular inclined planes; but the softer superincumbent argillaceous beds have been bent by the lateral pressure into many curious contortions, such as we formerly described in treating of the primitive and coal formations. It may indeed be admitted as a general fact, that when thick and compact beds of stone are associated with thin yielding beds, in a series of inclined strata, the stone, however much it may be raised, retains its flat planes of stratification, while the tabular clays are bent and twisted into the most irregular curves. This disposition affords demonstrative evidence of the beds having been upheaved and dislocated by mechanical violence after their horizontal deposition. The average height to which this formation rises is about 500 feet. The isle of Portland is 300. In general, these beds are nearly horizontal, where they have suffered no eruptive violence. But from this force, in the vale of Nadder, they are elevated sometimes to an angle of 45°; while at Upway and Portland they decline in opposite directions, forming a portion of what has been called the Weymouth saddle. See Geol. of Eng. Fig. p. 192. § X. BEDS BETWEEN THE CHALK AND OOLITE SERIES. This interval may be regarded as filled up by a series of beds chiefly of siliceous sand, which possess an aggregate thickness in the greater part of their course of about 1000 feet. They form the |