PURBECK BEDS. 
333 
in the same relative position in the cliffs east of Lulworth 
Cove, in Dorsetshire, where, as the strata have been dis¬ 
turbed, and are now inclined at an angle of 45°, the stumps 
of the trees are also inclined at the same angle in an opposite 
direction—a beautiful illustration of a change in the position 
of beds originally horizontal (see Fig. 310). 
Section of cliff east of Lulworth Cove. (Buckland and De la Beche.) 
From the facts above described we may infer, first, that 
those beds of the Upper Oolite, called the Portland,” which 
are full of marine shells, were overspread with fluviatile mud, 
which became dry land, and covered by a forest, throughout 
a portion of the space now occupied by the south of Eng¬ 
land, the climate being such as to permit the growth of the 
Zamia and Cycas. 2dly. This land at length sank down and 
was submerged with its forests beneath a body of fresh watef, 
from which sediment was thrown down enveloping fluviatile 
shells. 3dly. The regular and uniform preservation of this 
thin bed of black earth over a distance of many miles, shows 
that the change from dry land to the state of a fresh-water 
lake or estuary, was not accompanied by any violent denu¬ 
dation, or rush of water, since the loose black earth, together 
with the trees which lay prostrate on its surface, must inev¬ 
itably have been swept away had any such violent catastro¬ 
phe taken place. 
The forest of the dirt-bed, as before hinted, was not every¬ 
where the flrst vegetation which grew in this region. Be¬ 
sides the lower bed containing upright Cycadeoe^ 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. 
Subdivisions of the PurhecJc ,—It will be observed that the 
division of the Purbecks into upper, middle, and lower, was 
made by Professor Forbes strictly on the principle of the en- 
