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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 become fossil on the spots where they grew.
Besides the upright stumps above mentioned, the dirt-bed contains the stems of silicified trees laid prostrate. These are partly sunk into the black earth, and partly enveloped by a calcareous slate which covers the dirt-bed.
The dirt-bed contains exuviae of tropical trees, accumulated through time, as the forest shed its honours on the spot where it grew, and became itself decayed.
The great dirt-bed or vegetable soil containing the 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.
Immediately above the upper oolitic group in Buckinghamshire, in the vicinity of Weymouth, and other situations, there is a thin stratum, usually called by workmen the DIRT-BED, which appears, from incontestable evidence, to have been a soil, formed, like soils of the present day, in the course of time, upon a surface which had previously been the bottom of the sea.
Each dirt-bed may, no doubt, be the memorial of many thousand years or centuries, because we find that two or three feet of vegetable soil is the only monument which many a tropical forest has left of its existence ever since the ground on which it now stands was first covered with its shade.
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.
The forest of the dirt-bed, as before hinted, was not everywhere the first vegetation which grew in this region. Besides the lower bed containing upright Cycadeae, before mentioned, another has sometimes been found above it, which implies oscillations in the level of the same ground, and its alternate occupation by land and water more than once.
The diameter of these near the root is about one foot; but I measured one myself, in 1866, which was 3 1/2 feet in diameter, said by the quarrymen to be unusually large. Root-shaped cavities were observed by Professor Henslow to descend from the bottom of the dirt-bed into the subjacent fresh-water stone, which, though now solid, must have been in a soft and penetrable state when the trees grew.
We may, therefore, perceive distinctly that, as the pines and cycadeous plants of the ancient "dirt-bed," or fossil forest, of the Lower Purbeck were killed by submergence under fresh water, and soon buried beneath muddy sediment, so an invasion of argillaceous matter put a sudden stop to the growth of the Bradford Encrinites, and led to their preservation in marine strata.
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