23 



the positions they now occupy, viz. two simple downthrow 

 faults and two reversed upthrow faults, both hading in differ- 

 ent directions from each other ! and yet the Kimmeridge Clay 

 on each side the pit is undisturbed and nearly horizontal. 

 He naturally concludes that so extraordinary an arrangement 

 renders the fault theory in the highest degree improbable ; and 

 there remain only two possible hypotheses, viz. either there 

 has been a land-slip from a Chalk cliff overhanging a valley in 

 the Kimmeridge Clay, and the Chalk once on Koslyn Hill has 

 since been denuded away, or, the mass is an included Boulder, 

 and has been brought to its present position on an ice-raft. He 

 points out that Mr Seeley's section at the south end of the pit 

 {G. M. V. p. 348) is certainly wrong, that the Brown Sand 

 which with superjacent Gault is there shown between the 

 Kimmeridge Clay and Boulder Clay, is not in situ, but consists 

 only of numerous blocks of Neocomian Sandstone included in 

 the latter Clay. Mr Bonney says, "I can only explain the mode 

 in which these blocks occur by supposing that, before and 

 during the accumulation of the Boulder Clay there was, nearly 

 along the line of the south side of the pit, a cliff or bank of 

 Kimmeridge Clay, capped by Neocomian Rock, from which 

 fragments slipped and fell." 



He does not think however that any such slip will explain 

 the position of the Gault and Chalk ; he fails to see any 

 conformity between the Gault and Neocomian Sands in any 

 part of the pit, and considering that the Gault (with presumably 

 the Chalk and Greensand) rests now on Boulder Clay, now on 

 Kimmeridge Clay, and now on disturbed Neocomian Sand, he 

 concludes that the idea of these Cretaceous beds being a huge 

 fragment or transported mass in the Boulder Clay is more 

 in accordance with the observed facts, and that it presents far 

 less difficulty than either of the other theories. He only 

 differs from Mr Fisher in thinking that the valley existed 

 before the Boulder was dropped. 



- 1875. In his "Cambridgeshire Geology," Mr Bonney 

 describes the Post-pliocene deposits at p. 49 and quotes largely 

 from Mr Seeley's papers on the subject, which have been already 

 reviewed. 



Under the head of Boulder Clay Mr Bonney refers to a fresh 



