334 
ELEMENTS OF GEOLOGY. 
tire distinctness of the species of organic remains which they 
include. The lines of demarkation are not lines of disturb¬ 
ance, nor indicated by any striking physical characters or 
mineral changes. The features which attract the eye in the 
Purbecks, such as the dirt-beds, the dislocated strata at Lul- 
worth, and the Cinder-bed, do not indicate any breaks in the 
distribution of organized beings. “ The causes which led to 
a complete change of life three times during the deposition 
of the fresh-water and brackish strata must,” says this nat¬ 
uralist, “be sought for, not simply in either a rapid or a sud¬ 
den change of their area into land or sea, but in the great 
lapse of time which intervened between the epochs of depo¬ 
sition at certain periods during their formation.” 
Each dirt-bed may, no doubt, be the memorial of many 
thousand years or centuries, because we find that two or 
three feet of vegetable soil is the only monument which many 
a tropical forest has left of its existence ever since the ground 
on v/hich it now stands w^as first covered with its shade. 
Yet, even if we imagine the fossil soils of the Lower Purbeck 
to represent as many ages, we need not be sui;prised to find 
that they do not constitute lines of separation between strata 
characterized by different zoological types. The preserva¬ 
tion of a layer of vegetable soil, when in the act of being 
submerged, must be regarded as a rare exception to a gen¬ 
eral rule. It is of so perishable a nature, that it must usually 
be carried away by the denuding waves or currents of the 
sea, or by a river; and many Purbeck dirt-beds were proba¬ 
bly formed in succession and annihilated, besides those few 
which now remain. 
The plants of the Purbeck beds, so far as our knowledge 
extends at present, consist chiefly of Ferns, Coniferae, and 
Cycadeae (Fig. 308), without any angiosperms; the whole 
more allied to the Oolitic than to the Cretaceous vegetation. 
The same afiinity is indicated by the vertebrate and inverte¬ 
brate animals. Mr. Brodie has found the remains of beetles 
and several insects of the homopterous and trichopterous 
orders, some of which now live on plants, while others are 
of such forms as hover over the surface of our present rivers. 
Portland Oolite and Sand (^, Tab., p. 321).—The Portland 
Oolite has already been mentioned as forming in Dorsetshire 
the foundation on which the fresh-water limestone of the 
Lower Purbeck reposes (see p. 331). It supplies the well- 
known building-stone of which St. Paul’s and so many of the 
principal edifices of London are constructed. About fifty 
species of mollusca occur in this formation, among which are 
some ammonites of large size. The cast of a spiral univalve 
