PURBECK BEDS. 
331 
Cyprides from the Lower Purbeck. 
a. Ciipris Purbeckensis, Forbes, b. 
Same magnilied. c. Cypris punc¬ 
tata, Forbes, d, e. Two Views mag¬ 
nified of the same. 
Middle Purbeck. This is the be- Fig.sor. 
ginning of the inferior division, 
which is about 80 feet thick. 
Below the marls are seen, at 
Meup’s Bay, more than thirty 
feet of brackish - water strata, 
abounding in a species of Ser- 
pula^ allied to, if not identical 
with, Serpula coacervites^ found 
in beds of the same age in Hanover. There are also shells of 
the genus Missoa (of the subgenus and a little Car- 
dium of the subgenus Protocardiiim^ in these marine beds, to¬ 
gether 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 Cycadece,^ which I shall presently de¬ 
scribe, underlies these marls, and rests upon the lowest fresh¬ 
water limestone, a rock about eight feet thick, containing Cy- 
das,, Valvata^ and Limncea,, 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 ujDon the 
top beds of the Portland stone, which, although it contains 
purely marine remains, often consists of a rock undistin- 
guishable in mineral character from the Lowest Purbeck 
limestone. 
Dirt-bed or ancient Surface-soil. —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 color, and con¬ 
tains a large proportion of earthy 
lignite. Through it are dispersed 
rounded and sub-angular fragments 
of stone, from 3 to 9 inches in diame¬ 
ter, 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 nidi- 
ba°es^ofas those found in the upper 
bed,but no Coniferm, The weight of 
the incumbent strata.squeezing down the compressible dirt- 
bed has caused the Cycads to assume that form which has 
Fig. 803. 
Mantellia nidiformis, Brongni- 
art. The upper part shows the 
