Excerpt: ...roots and stools of Cycadeae, which I shall presently describe, underlies these marls, and rests upon the lowest fresh-water limestone, a rock about eight feet thick, containing Cyclas, Valvata, and Limnaea, of the same species as those of the uppermost part of the Lower Purbeck, or above the dirt-bed. The fresh-water limestone in its turn rests upon the top beds of the Portland stone, which, although it contains purely marine remains, often consists of a rock undistinguishable in mineral character from the ...
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Excerpt: ...roots and stools of Cycadeae, which I shall presently describe, underlies these marls, and rests upon the lowest fresh-water limestone, a rock about eight feet thick, containing Cyclas, Valvata, and Limnaea, of the same species as those of the uppermost part of the Lower Purbeck, or above the dirt-bed. The fresh-water limestone in its turn rests upon the top beds of the Portland stone, which, although it contains purely marine remains, often consists of a rock undistinguishable in mineral character from the Lowest Purbeck limestone. DIRT-BED OR ANCIENT SURFACE-SOIL. (FIGURE 308. Mantellia nidiformis, Brongniart. The upper part shows the woody stem, the lower part the bases of the leaves.) 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 colour, and contains a large proportion of earthy lignite. Through it are dispersed rounded and sub-angular fragments of stone, from 3 to 9 inches in diameter, in such numbers that it almost deserves the name of gravel. I also saw in 1866, in Portland, a smaller dirt-bed six feet below the principal one, six inches thick, consisting of brown earth with upright Cycads of the same species, Mantellia nidiformis, as those found in the upper bed, but no Coniferae. The weight of the incumbent strata squeezing down the compressible dirt-bed has caused the Cycads to assume that form which has led the quarrymen to call them "petrified birds' nests," which suggested to Brongniart the specific name of nidiformis. I am indebted to Mr. Carruthers for Figure 308 of one of these Purbeck specimens, in which the original cylindrical figure has been less distorted than usual by pressure. Many silicified trunks of coniferous trees, and the remains of plants allied to Zamia and Cycas, are buried in this dirt-bed, and must have...
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