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a dozen other mollusca. They occur very near the bottom of 
the sands, heretofore called the Carstone. 
These facts do not demonstrate the position in the geological 
sequence of the Potton and Wicken beds. 
In the south of England the beds between the Kimeridge 
clay and the galt are 
(Lower Green or) North-Down sands*, 
Wealden series, 
Purbeck series, 
Portland series, 
and to represent the whole of these there are in this district only 
the Potton sands. These sands in the middle of England have 
generally been referred to some portion of the series. Thus 
Smith called them the sand of the Portland rock. Conybeare, 
who instituted the Ironsand group, supposed the Portland series 
as well as the Greensand to thin away northward, and so put 
these deposits into his Ironsands; and Fitton, who instituted 
the Lower Greensand, supposed the Portland, the Purbeck, and 
the Weald, to thin off te the north, and threw these beds into 
the Lower Greensand. 
The author then detailed at length the physical characters of 
the beds between the Kimeridge clay and the Galt in all the 
English sections, and arrived at the conclusion that the period 
of elevation indicated by the pebble beds of Potton and Wicken 
was identical with the period of elevation indicated in the south 
of England by the Purbeck and Wealden group, the marine 
equivalents of which would be thin. 
And in this district the author supposed the upper part of 
the clay called Kimeridge to be only the necessary clay repre- 
sentative of beds which to the south are sands. 
Under these circumstances it was thought that the old 
* The North Downs give the types of the divisions of the so-called Lower 
Greensand adopted by the Geological Survey. 
