296 



LOWER PURBECK. 



[Ch. XX 



Cyprides from the Lower Purbecks. 



&. CyprU punctata^, 

 E. Forbes. 



Beneath the freshwater strata hist described, a very thin band o. 

 greenish shales, with marine shells and impressions of leaves, like those 

 of a large Zostera,, succeeds, forming the base of the Middle Purbeck. 



Lower Purheck. — Beneath the thin marine band above mentioned, 

 purely freshwater marls occur, containing species of Ct/pris (fig. 339, 

 a, 5), Valvata, and Lymnmus, dif- 

 ferent from those of the Middle 

 Purbeck. This is the bemnniuo' 

 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 a. Ci/pns Purbecke^isis, 

 with, Serpula coacervites, found in 

 beds of the same ao-e in Hanover. There are also shells of the afenus 

 Rissoa (of the subgenus Jli/drohia), and a little Cardium of the sub 

 genus Protocmxlium,\n the same beds, together with Cijpris. 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 Cycadece, which I shall presently 

 describe, underlies these marls, and rests upon the lowest freshwater 

 limestone, a rock about 8 feet thick, containing CycJas, Valvata, and 

 Limnceus, 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 marine remains, often consists of a rock 

 quite homogeneous in mineral 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 ^.^ g^^ 



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 lai'ge proportion of 

 earthy lignite. Through it are 

 dispersed rounded fragments of 

 stone, from 3 to 9 inches in diame- 

 ter, in such numbers that it almost 

 deserves the name of gravel. Many 

 silicified trunks of coniferous trees, and the remains of plants allied tu 

 Zamia and Cycas, are buried in this dirt-bed (see figure of fossil species, 

 fig. 340, and of living Zamia, fig. 341). 



* Weston, Geol. Q. J., vol viii. p. IIY. 



^/^5^i 



C'ycadeoidea [Ifantellia) megalophyUa, 

 Bucklaad. 



