GEOLOGICAL RESULTS. 
335 
periods ; the lowermost or most ancient of which 
(as we have already noticed) is of fluviatile origin, 
and re])oses on the Oolite^ a marine formation of 
great extent, that forms an important feature in 
the physical structure, not only of England, but 
also of the C'ontinent. ddie Portland Limestone 
constitutes the uppermost division of the Oolite, 
and contains marine remains only ; it is succeeded 
by the fresh-water strata of the Isle of Parheck^ 
which may be considered as the lowermost de])o- 
sits of the AVealdcn ; for although the Purbeck 
marble, comjiosed of a small species of paludina, 
has not been observed within the wealds of Kent 
and Sussex, yet the shales and limestones of Ash- 
burnham and Brinlitlinc; bear a close resemblance 
to some of the slaty strata of Purbeck, and the 
organic remains in all are of the same fluviatile 
character. 
But there is a fact connected with the history of 
the Portland and Purbeck beds, so highly interest- 
ing, and which illustrates in so striking a manner 
the nature of one of those grand geological muta- 
tions which have taken place in the south of Eng- 
land, that it will be necessary to notice it here, 
although it occurs without the limits of the district, 
which it is the professed object of this work to 
describe. 
In the island of Portland, the oolitic limestone is 
extensively quarried for architectural purposes, and 
su])plies most of the cities and towns in the south- 
east of England. On these oolitic strata are placed 
deposits of a totally different character. Immedi- 
ately on the uppermost marine stratum (which 
