372 Mr. Lyell on the Boulder Formation^ 



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: 

 Purimra crispata^ Fusus strialus and contrarius, Littorina 

 littorea and squalida, Scalaria grcenlandica, Natica helicoides, 

 Nucida Cobboldice, Cardium edule^ Cyprina islandica, Tellina 

 obliqua, T. soli dida, and Mya areiiaria? Ail of these are 

 known as recent except three, Fusus st?'iatus, Tellina obliqua, 

 and Nucida 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 helicoidcs, 

 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. 



