. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 380 MIDDLE PUBBECK. [Ch. XX The stone called " Purbeck marble," formerly much used in ornamental architecture in the old English cathedrals of the southern counties, is ex- clusively procured from this division. Middle Purbeck.—Next in succession is 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 pre- ceding strata. Below the limes
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 380 MIDDLE PUBBECK. [Ch. XX The stone called " Purbeck marble," formerly much used in ornamental architecture in the old English cathedrals of the southern counties, is ex- clusively procured from this division. Middle Purbeck.—Next in succession is 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 pre- ceding strata. Below the limestone are brackish-water beds full of Cyrena, and traversed by bands abounding in Corbula and Melania. These are based on a purely marine deposit, with Pecten, Modiola, Avicula, Thracia, all undescribed shells. Below this, again, come lime- stones 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 crocodilian reptile named Macrorhyncus. Among the mollusks, a remarkable ribbed Melania, of the section Chilina, 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. 369). In the uppermost part of this bed Professor Forbes discovered the first echino- derm (fig. 370) as yet known in the Purbeck series, a species of Ilemici- daris, a genus characteristic of the Oolitic period, and scarcely, if at all, distinguishable from a previously known oolitic species. It was accom- Fig. 369. Fig. Ostrea distorta. Cinder-bed, Middle Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Lyell, Charles, Sir, 1797-1875. New York, D. Appleton and co.
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