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 become fossil on the
spots where they grew.
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