a. portion of the same magnified.
From such facts we may infer that, notwithstanding the great thickness of this division of the Wealden (and the same observation applies to the Weald Clay and Purbeck Beds), the whole of it was a deposit in water of a moderate depth, and often extremely shallow. This idea may seem startling at first, yet such would be the natural consequence of a gradual and continuous sinking of the ground in an estuary or bay, into which a great river discharged its turbid waters. By each foot of subsidence, the fundamental rock, such as the Portland Oolite, would be depressed one foot farther from the surface; but the bay would not be deepened, if newly deposited mud and sand should raise the bottom one foot. On the contrary, such new strata of sand and mud might be frequently laid dry at low water, or overgrown for a season by a vegetation proper to marshes.
Purbeck Beds.
Immediately below the Hastings Sands we find a series of calcareous slates, marls, and limestones, called the Purbeck Beds, because well exposed to view in the sea-cliffs of the Peninsula of Purbeck, especially in Durlestone Bay, near Swanage. They may also be advantageously studied at Lulworth Cove and the neighbouring bays between Weymouth and Dorchester. At Meup's Bay in particular, Prof. E. Forbes has recently examined minutely the organic remains of the three members of the Purbeck group, displayed there in a vertical section 155 feet thick. To the information previously supplied in the works of Messrs. Webster, Fitton, De la Beche, Buckland, and Mantell, he has made most ample and important additions, so that it will be desirable to give them at some length, it appearing that the Upper, Middle, and Lower Purbecks are each marked by peculiar species of organic remains, these again being different, so far as a comparison has yet been instituted, from the fossils of the overlying Hastings Sands and Weald Clay. This result cannot fail to excite much wonder, and it leads us to suspect that the Wealden period, which many geologists have scarcely deigned to notice in their classification, may comprehend the history of a lapse of time as great as that of the Oolitic or Cretaceous eras respectively.[231-A]
Upper Purbeck.—The highest of the three divisions is purely freshwater, the strata, about 50 feet in thickness, containing shells of the genera Paludina, Physa, Lymnea, Planorbis, Valvata, Cyclas, and Unio, with cyprides, and fish.
Middle Purbeck.—To these succeed the Middle Purbeck, about 30 feet thick, the uppermost part of which consists of freshwater limestone, with cyprides, turtles, and fish of different species from those in the preceding strata. Below the limestone are brackish-water beds full of Cyrena, and traversed by bands abounding in Corvulæ and Melaniæ. These are based on a purely marine deposit, with Pecten, Modiola, Avicula, and Thracia, all undescribed shells. Below this, again, come limestones and shales, partly of brackish and partly of freshwater origin, in which many fish, especially species of Lepidotus and Microdon radiatus, are found, and a reptile named Macrorhyncus. Among the mollusks, a remarkable ribbed Melania, of the section Chilira, occurs.
Immediately below is the great and conspicuous stratum, 12 feet thick, long familiar to geologists under the local name of "Cinder-bed," formed of a vast accumulation of shells of Ostrea distorta ([fig. 240.]). In the uppermost part of this bed Mr. Forbes discovered the first echinoderm as yet known in the Purbeck series, a species of Hemicidaris, a genus characteristic of the Oolitic period. It was accompanied by a species of Perna. Below the Cinder-bed freshwater strata are again seen, filled in many places with species of Cypris, Valvata, Paludina, Planorbis, Lymnea, Physa, and Cyclas, all different from any we had previously seen above. Thick siliceous beds of chert, filled with these fossils, occur in a beautiful state of preservation, often converted into chalcedony. Among these Mr. Forbes met with gyrogonites (the spore vesicles of Charæ), plants never before discovered in rocks older than the Eocene. Again, beneath these freshwater strata, a very thin band of greenish shales, with marine shells and impressions of leaves, like those of a large Zostera, succeeds, forming the base of the Middle Purbeck.
Fig. 240.
