194 
DEPOSITS OF DOUBTFUL AGE. 
The evidence on which the different isolated sections have 
been correlated is not satisfactory. Besides the want of 
continuity of the deposits, there is the difficulty, first pointed 
out to me by Prof. Otto Torell, that the bed of oysters at 
Sherringham can scarcely be of the same age as the gravel with 
Leda my alls and Astarte borealis at E unton. The oyster is 
sensitive to cold, and does not now live in the seas that support 
these arctic mollusca. Oysters are, however, abundant in the 
Nar Valley Beds associated with certain arctic species. It is 
possible that the different sections of the Leda-myalis Bed 
are fragments of the marine deposits belonging to a period of 
considerable length, during which the depth of water varied and 
the climate changed. The alternate piling up and scouring 
away of sandbanks would cause sometimes one, sometimes 
another horizon to rest on the Forest-bed—^just as we have seen 
that different zones of Ked Crag may overlap on to the London 
CJay, 
From its unconformity with the underlying and overlying fresh- 
w^ater strata, and from the small fauna yet obtained, it is impossible 
at present to say definitely whether the Leda-myalis Bed is more 
allied to the Crag or to the Glacial deposits. Of the fossils it is 
only safe to take into account those that seem undoubtedly to 
belong to the deposit. The reversed Trophon antiquus and 
Tellina ohliqua occur, unfortunately, so rarely, and under such 
circumstances, that there is a suspicion of their being derived 
from the underlying Forest-bed or Weybourn Crag. Only 
about ten species of mollusca are undoubtedly contemporaneous 
with the deposit. One of these, Leda myalls, is an arctic species 
unknown in the Crag.* Another, Ostrea edulis, is not found in 
the "Weybourn and Chillesford (h’ag, though common lower 
down. 
No additions have been made to the fauna of this deposit for 
several years, but till further evidence is obtained, it seems most 
convenient to include the Leda-myalis Bed in the Pliocene 
Series—as was done in the Cromer Memoir. 
The Arctic Freshwater Bed must certainly be classed with the 
Pleistocene deposits. It is described here, because of the great 
difficulty found in separating it from the Leda-myalis Bed, and 
from the Forest-bed, in the absence of fossils. It also helps to 
connect stratigraphically the Pliocene and Pleistocene formations ; 
but its fossils, so far as known, differ so completely from those of 
the Forest-bed, and are so thoroughly Arctic, that there must 
be a considerable palaeontological break between the two horizons. 
During a visit to England in the year 1872, Prof. A. G. 
Nathorst, of the Geological Survey of Sweden, discovered, 
immediately under the Till at Mundesley, a bed of clay and 
* All the sj5ecimens found in the Crag belong to Leda ohlongoides of Wood, not 
to Leda myalis. The latter seems to be unknown on this side of the Atlantic, 
except in the Leda-mijalis Bed. 
