_Lower Purbeck._--Beneath the thin marine band last mentioned, purely freshwater marls occur, containing species of _Cypris_, _Valvata_, and _Lymnea_, different from those of the Middle Purbeck. This is the beginning of the Inferior division, which is about 80 feet thick. Below the marls are seen more than 30 feet of brackish-water beds, at Meup's Bay, abounding in a species of _Serpula_, allied to, if not identical with, _Serpula coacervites_, found in the Wealden of Hanover. There are also shells of the genus _Rissoa_ (of the subgenus _Hydrobia_), and a little _Cardium_ of the subgenus _Protocardium_, in the same beds, together with _Cypris_. Some of the cypris-bearing shales are strangely contorted and broken up, at the west end of the Isle of Purbeck. The great dirt-bed or vegetable soil containing the roots and stools of _Cycadeae_, which I shall presently describe, underlies these marls, resting upon the lowest freshwater limestone, a rock about 8 feet thick, containing _Cyclades_, _Valvata_, and _Lymnea_, of the same species as those of the uppermost part of the Lower Purbeck. This rock rests upon the top beds of the Portland stone, which is purely marine, and between which and the Purbecks there is no passage. The most remarkable of all the varied successions 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 fragments 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 living _Zamia_, fig. 241.). These plants must have become fossil on the spots where they grew. The stumps of the trees stand erect for a height of from 1 to 3 feet, and even in one instance to 6 feet, with their roots attached to the soil at about the same distances from one another as the trees in a modern forest.[233-A] The carbonaceous matter is most abundant immediately around the stumps, and round the remains of fossil _Cycadeae_.[233-B] [Illustration: Fig. 241. Zamia spiralis; Southern Australia.[233-C]]