TRANSACTIONS OF THE SECTIONS. 81 
mollusks a remarkable ribbed Melania, of the section Chilina, is found here. After 
the deposition of these strata, there came another powerful influx of the sea, intro- 
ducing marine species—Pectens, Modiola, Avicula and Thracie—all undescribed 
forms. Brackish-water strata full of Cyrena, and traversed by bands abounding in 
Corbule and Melanie, are next in order ; in them is a Profocardia, but quite distinct 
from its representative species in the lower portion of the Purbeck limestones. 
Cyprides, turtles and fish, crown these brackish-water bands, and are specifically 
connected with the beds of the middle Purbecks below them. 
Lastly, a third series of freshwater strata commence with a new series of fossils— 
Cyprides, Paludine, Physa, Limneus, Planorbis, Valvata, Cyclades and Unio, and 
fresh forms of fish. These continue until they merge into the base of the Hastings 
sands, and the Purbeck series is completed. The total thickness of all the Purbecks 
at Meup’s Bay is about 155 feet. Of this one-half is occupied by the lower portion 
of the series, and the remainder is divided between the middle and upper portions, 
the former being rather the more extensive. At Swanage the thickness is greater. 
It is very remarkable, that whilst we can strictly divide the Purbecks into upper, 
middle and lower, each marked by a peculiar assemblage of organic remains, the lines 
of demarcation between these sections are not lines of disturbance, nor indicated by 
striking physical characters or mineral changes. The features which attract the eye 
in the Purbecks, such as the dirt-beds, the dislocated strata at Lulworth 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 freshwater and brackish strata of the Purbeck series, must be sought for, not 
simply in either a rapid or a sudden change of their area into land or sea, but in the 
great lapse of time which intervened between the epochs of deposition at certain pe- 
riods during their formation. A most striking feature of the molluscan fauna of the 
Purbecks is this—so similar are the generic types of these mollusca to those of tertiary 
freshwater strata and those now existing, that, had we only such fossils before us, 
and no evidence of the infraposition of the rocks in which they are found, we should 
be wholly unable to assign them a definite geological epoch. An examination and 
comparison of these Purbeck fossils with the collections from the Hastings sands and 
Wealden in the Museum of the Geological Society (to which they were chiefly pre- 
sented by Dr. Fitton), and in the cabinet of Dr. Mantell, leads the author to believe 
that the fauna of the middle and upper divisions of the Wealden series is, so far as 
species are concerned, almost entirely distinct from that of the lower or Purbeck di- 
vision. Many of the species reputed to be common to the whole series, are found 
on inquiry to include more species than one under one name; whilst some other 
forms recorded as Wealden, but so far as the author has observed, peculiar to the 
upper Purbecks, and occupying only a limited horizon in that part of the series, are 
derived from certain anomalous strata near Tunbridge Wells, which the author be- 
lieves will prove, on closer examination and accurate survey, to be Purbeck strata, 
brought up among the Wealden clays by faults. The excellent monograph on the 
Wealdens of North Germany, by Dunker and Von Mayer, in which a vast number 
of species of animals and plants are described and accurately figured, affords the 
strongest confirmation of this view, and shows that while the faunas of the German 
Weald clays and Hastings sands correspond in essentials with that of the same for- 
mations in Britain, the Purbecks of the continent, just as here, differ from the su- 
perior beds almost entirely in their organic contents, and correspond with similar 
beds in our own series. 
The marine or brackish-water bands in Germany containing Ostrea Fittoniana, 
appear to be represented in England by corresponding bands with the same fossil, 
accompanied by species of Corbula, Cardium and Melania, in the upper part of the 
Hastings sand at Swanage. All the investigations of the author, so far, have gone 
to indicate the probability of the presence of several distinct assemblages of organic 
remains (similar to those which he has shown to exist in the Purbecks) in the higher 
portions of the Weaiden series of formations, whilst the true position of the strata, 
‘now described, is shown without a question to be in connection with the oolitic or 
lower, and not with the cretaceous or upper division of the secondary rocks. 
~ 1850, @ 
