372 Mr. Lyell on the 'Boulder Pormation^ 
Norwich crag, which is rarely more than one or two feet thick, 
except near Weybourne. Upon the crag, and where this is 
wanting immediately upon the chalk, rests here and there a 
lignite and freshwater formation, which varies in thickness 
from five to ten feet and upwards. It is seen at intervals 
throughout the whole line of cliff from Hasborough to Run- 
ton. In some places it resembles a bed of lignite, in others 
a black earth like that found in connexion with peat, while 
occasionally it consists of gravel, sand, clay, and marl, such 
as may be met with in any lacustrine deposit. In certain 
localities it contains the stools of trees, which remain in the 
position in which they originally grew, and which could only 
have been buried under the strata now incumbent on them 
by the submergence of what was once dry land. At Mun- 
desley the freshwater formation is about 40 feet thick and 
occupies the whole cliff. 
As both the crag and freshwater formations are extremely 
discontinuous in the mud cliffs, we sometimes find the one 
and sometimes the other in immediate contact with the chalk, 
while in many places both are wanting, and then the chalk 
is covered exclusively by drift, of which the great mass of 
the mud cliffs is composed. A cursory observer, indeed, 
might see nothing but drift from Hasborough to Cromer, 
except at Trimmingham, where the protuberances of chalk 
occur ; and the section north of Cromer would seem to pre- 
sent little more than the same drift, with a slight exposure of 
chalk on the sea beach. The thin stratum of freshwater 
origin and the subjacent marine crag are most commonly 
hidden by the beach, or by the sea, except at low water. 
Age of the crag. — As to the age of the crag, it agrees with 
that of Norwich in the species of marine shells which it con- 
tains, and the occasional presence of land shells and the 
rolled bones of mammalia. From the various localities above 
enumerated, I obtained the following eleven species of shells : 
Purpura crispata^ Fusus striatus and contrarms, Littorma 
littorea and squalida, Scalaria groenlandica, Natica helicoideSy 
Nucida Cobholdice, Cardium edule^ Cyprina isla7idica^ Tellina 
obliqua, T. solidula^ and Mya arenaria ? All of these are 
known as recent except three, Fusus striatus^ Tellina obliqtia, 
and Nucula Cobboldice. It would be rash however to pre- 
tend to determine the per centage of recent species from so 
small a number, and the late discovery of Natica helicoides.^ 
one of the eleven, in a living state, should make us careful 
not to assume, when reasoning on these more modern de- 
posits, that we have acquired a perfect acquaintance with the 
present Fauna of our seas. 
