consists of limestones, sands, and clays of great thickness and extent, which abound in marine shells, corals, crinoidea, articulata, and fishes-terrestrial and marine reptiles, insects, and land plants-and two or three genera of very small mammalia, referable to the marsupialia and insectivora. In this formation there are several local intercalations of fluviatile deposits, resembling the Wealden in the presence of terrestrial animals and plants, and freshwater shells.* Certain beds of the oolite (called the Stonesfield slate) contain organic remains, which, with the exception of trigoniæ and other marine shells, correspond with those of the Tilgate calciferous grits. The collocation of teeth and bones of the Megalosaurus, Lepidotus, &c., with stems and fruits of cycadeous plants, presented so striking an analogy, that it led me, on my first discovery of similar fossils in the Tilgate strata, to seek for the equivalent of the latter in the Stonesfield oolite. In the Isle of Purbeck, the uppermost group of the oolite only appears; it is denominated "the Portland Oolite," from the island in which the limestone has for centuries been extensively quarried. This group is separated into three divisions, viz.: 1, the Portland limestones; 2, the Portland sands; and, 3, the Kimmeridge clays, so called, from the bay of Kimmeridge, where these beds are well displayed.

1. PORTLAND LIMESTONES.-Oolitic limestone, abounding in trigoniæ, ammonites, pernæ, plurotomariæ, and other marine shells; layers and nodular masses of chert and flint; total thickness, from 30 to 40 feet.

2. PORTLAND SANDS.-Beds of white and green sand and sandstone, with concretionary masses of grit; the lower strata argillaceous; total thickness, about 130 feet.

*See "Wonders of Geology," vol. ii. p. 436.

See "The Fossils of the South Downs," 1822, "On the analogy between the organic remains of the Tilgate beds and those of Stonesfield, near Oxford," p. 59.

3. KIMMERIDGE CLAY.-Dark blue clay, with septaria, and bands of sandy concretions; beds of lignite, and highly bituminous shales and clays; marine shells are very numerous; total thickness, 300 feet. One species of oyster, the Ostrea deltoidea, is characteristic of this group, and occurs abundantly in every locality.*

GEOGRAPHICAL DISTRIBUTION OF THE STRATA.—The White chalk forms a narrow but lofty chain of hills, that ranges inland across the island, from Handfast Point (section, lign. 28) to the cliff called Bat's Corner, on the southwestern shore. The lower cretaceous deposits appear along the southern escarpment of the downs; the chalk is also exposed to the east of Bat's Corner, in Worbarrow, or Worthbarrow Bay, and in several lesser bays or coves that have been excavated, by the long-continued action of the waves, in the Purbeck and Wealden deposits, that form the southern cliffs. The height of this chain of downs is from 500 to between 600 and 700 feet. Ballard Hill, over Handfast Point, is 584 feet, and Ninebarrow Down, 642 feet high. A line of chalk cliffs forms the coast from Bat's Corner to Whitenore (see lign. 36), where the cretaceous beds trend inland, and the cliffs beyond consist of the strata below the chalk. The Wealden and Hastings deposits extend entirely across the peninsula, from Swanage Bay to the southern shore at Worbarrow Bay, and form a valley, diversified with heaths and cultivated tracts, between the chalk escarpment and the next range. The Purbeck beds run in a nearly parallel direction with the Wealden, from the hills above Swanage, spreading out to the north as far as the village of Worth, and westward four or five miles, to the seashore on the south of Worbarrow Bay, and the adjacent coves.

*Sowerby's "Mineral Conchology" contains figures of many of the shells of the Portland oolite; and in Dr. Fitton's Memoir, plates xxii. and xxiii., several species are figured and described.

The Portland group appears on the south-east beneath the Purbeck at Durlstone-head, and continues along the coast to Gad-cliff, constituting the regular tract that forms the south-eastern part of the Isle of Purbeck.

SOUTHERN COAST OF PURBECK.-We must now take a rapid view of the most important features of the coast, from Durlstone-head to the western extremity of this interesting district. Swanage will be the most convenient place for the visitor desirous of exploring the geological phenomena of the eastern part of Purbeck; the numerous quarries in the vicinity, and the coast sections, being rich in organic remains. East and West Lulworth, two little villages near Worbarrow Bay, are the best halting-places for a survey of the south-western shores; and if the observer be tempted to extend his tour to the Isle of Portland, Weymouth, of course, will be the place of sojourn.

As the general dip of the strata is to the north, the lower beds are displayed along the southern coast; and we will now point out the most instructive localities, in proceeding from the south-eastern corner of the Isle of Purbeck, to the west.

CONTORTED PURBECK STRATA.-The strata in the middle and on the south side of Durlstone Bay, are very remarkable for their broken and contorted condition, which was evidently produced when the beds were in a plastic state. Septaria, veins of calcareous spar, and crystals of gypsum, occur in some of the clays; there are also masses of breccia, or conglomerate composed of fragments of the undulated layers, cemented together into a firm rock.*

To the west of Durlstone Head, the Portland oolite appears; and at Tillywhim (lign. 28) there are quarries for the extraction of the fine white limestone, here called Purbeck stone. These strata contain ammonites, trigoniæ,

* Mr. Webster has figured a mass o this kind in pl. xxxi. Sir H. Englefield's "Isle of Wight."

and the usual Portland shells, and veins and nodular masses of chert. A bed of oyster shells, which an infiltration of calcareous matter has converted into a hard shelly conglomerate, like that of Bromley, in Kent,* occurs in the upper part of the series. The quarries at Tilly whim are generally visited by parties in the summer, on account of the romantic character of the scenery, and the delicious cool retreat afforded by the caverns or excavations; these are reached by a path that winds through a deep valley, strewn with masses of stone, and the descent is by steps cut in the rock. To the eastward there are similar caves, formed by the extraction of the layers of limestone : fine specimens of the oyster-shell conglomerate may be obtained from near the summit of the cliffs.

Lofty perpendicular cliffs of oolite, capped with Purbeck strata, continue along the coast to the west; and at the distance of about three miles from Tilly whim, the Portland stone is worked in Windspit quarries, which are excavations in the cliff, like those at the former place.

About a mile to the west of Windspit quarries, the cliffs form a bold cape, called St. Adhelm's Head; and about midway between these places, the Portland sands emerge from beneath the oolitic limestones, and gradually rise towards the upper part of the cliff, as we proceed westward. The promontory of St. Adhelm's Head is a conspicuous feature of this part of the country, and is flanked by dangerous rocks, which have often proved fatal to mariners. It is a vertical section of the Portland limestone and sands, and of the underlying Kimmeridge strata; the former appearing at the top in immense square masses, and the latter below in a steep slope, covered by

* See "Medals of Creation," vol. i. p. 385.

† A view of the cliff at Tillywhim is given by Mr. Webster, in Sir H. Englefield's "Isle of Wight," pl. xxxiii.

Represented in pl. xxxii. of the same work.

huge fallen blocks and fragments of stone. Deep fissures in the upper part of the cliff predict the impending ruin of other masses, and the agitation of the sea for above a mile from the shore indicates the nature of its rocky bottom; the remains, no doubt, of land which has been overwhelmed by the ocean. On the summit of this precipice are the remains of St. Adhelm's Chapel, a small building of early Saxon architecture, originally a chauntry, where masses were performed for the safety of the mariners that passed by this dangerous shore, who were expected to leave some gratuity at the first port they landed at, for the maintenance of the priest.*

From this point to Gad Cliff, which, in direct distance, is about six miles, the coast is much indented, and is flanked by cliffs of Kimmeridge clay, which are capped along the middle district by Portland sand and limestone; the total thickness of the clays is about 300 feet.

Rounding St. Adhelm's Head, the coast tends northward, and a creek, called Chapman's Pool, appears on the right; beyond is Emmett's Hill, a ridge nearly 300 feet high, capped with Portland stone, resting on the Portland sands, of which a good section is here exposed.† To the west, the cliffs for between two and three miles are, in a great measure, composed of highly bituminous shales, and laminated clays, termed Kimmeridge coal;" we then reach Kimmeridge Bay, an excavation formed by the sea, in the low tract of fissile argillaceous deposits which constitutes this part of the coast. The view of Gad Cliff from Kimmeridge Bay is very imposing, for in the foreground the black bituminous clay cliffs run out into a low point, and, towering over them to the westward, the limestone summit of the distant hill is distinctly visible. The bay is partly encircled by an amphitheatre of high ground, the Mr. Webster.

† See Dr. Fitton's "Memoir," p. 211.