Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 390 FOSSIL FORESTS IN ISLE OF PORTLAND [Oh. XX containing Cyclas, Valvata, and Zimncea, of the same species as those of the uppermost part of the Lower Purbeck, or above the dirt-bed. The freshwater limestone in its turn rests upon the top beds of the Portland stone, which, although it contains purely ma- rine remains, often consists of a rock quite homogeneous in min- eral character with the Lowest Purbeck limeston
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 390 FOSSIL FORESTS IN ISLE OF PORTLAND [Oh. XX containing Cyclas, Valvata, and Zimncea, of the same species as those of the uppermost part of the Lower Purbeck, or above the dirt-bed. The freshwater limestone in its turn rests upon the top beds of the Portland stone, which, although it contains purely ma- rine remains, often consists of a rock quite homogeneous in min- eral character with the Lowest Purbeck limestone.* The most remarkable of all the varied succession of beds enumerated in the above list, is that called by the quarrymen 'the dirt,' or 'black dirt,' which was evidently an ancient vegetable soil. It is from 12 to 18 inches thick, is of a dark brown or black color, and contains a large proportion of earthy Through it are dispersed rounded frag- ments of stone, from 3 to 9 inches in diameter, in such numbers that it almost deserves the name of gravel. Many silicified trunks of coniferous trees, and the remains of plants allied to Zamia and Cycas, are buried in this dirt-bed (see figure of fossil species, fig. 376, and of living Zamia, fig. 377). Fig. 37T. Cycadeoidea {MantelUa) megalophylla, Bucldand.
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