PURBECK BEDS. 
323 
It will be remarked, that the lines of steep slope, or escarp¬ 
ment, face towards the west in the great calcareous emi¬ 
nences formed by the chalk and the Upper, Middle, and 
Lower Oolites; and at the base of which we have respect¬ 
ively the Gault, Kimmeridge clay, Oxford clay, and Lias. 
This last forms, generally, a broad vale at the foot of the 
escarpment of inferior Oolite, but where it acquires consid¬ 
erable thickness, and contains solid beds of marlstone, it oc¬ 
cupies the lower part of the escarpment. 
The external outline of the country which the geologist 
observes in travelling eastward from Paris to Metz, is pre¬ 
cisely analogous, and is caused by a similar succession of 
rocks intervening between the tertiary strata and the Lias; 
with this difference, however, that the escarpments of Chalk, 
Upper, Middle, and Lower Oolites face towards the east in¬ 
stead of the west. It is evident, therefore, that the denud¬ 
ing causes (see p. 105) have acted similarly over an area 
several hundred miles in diameter, removing the softer clays 
more extensively than the limestones, and causing these last 
to form steep 'slopes or escarpments wherever the harder 
calcareous rock was based upon a more yielding and de¬ 
structible formation. 
UPPER OOLITE. 
Purbeck Beds. —These strata, which we class as the upper¬ 
most member of the Oolite, are of limited geographical ex¬ 
tent in Europe, as already stated, but they acquire impor¬ 
tance when we consider the succession of three distinct sets 
of fossil remains which they contain. Such repeated changes 
in organic life must have reference to the history of a vast 
lapse of ages. The Purbeck beds are finely exposed to view 
in Durdlestone Bay, near Swanage, Dorsetshire, and at Lul- 
worth Cove and the neighboring bays between Weymouth 
and Swanage. At Meup’s Bay, in particular. Professor E. 
Forbes examined minutely, in 1850, the organic remains of 
this group, displayed in a continuous sea-cliff section, and 
it appears from his researches that the Upper, Middle, and 
Lower Purbecks are each marked by peculiar species of or¬ 
ganic remains, these again being different, so far as a com¬ 
parison has yet been instituted, from the fossils of the over- 
lying Hastings Sands and Weald Clay. 
Tipper Purbeck, — The highest of the three divisions is 
purely fresh-water, the strata, about fifty feet in thickness, 
containing shells of the genera Paludina^ Physa,^ Pimnwa^ 
Planorhis^ Valvata^ Cyclas,, and Unio,, with Gyprides and fish. 
All the species seem peculiar, and among these the Gyprides 
