1848.] PRESTWICH ON STRATA OF CHRISTCHURCH HARBOUR. 4/ 



thickness has increased to about three feet ; two miles westward of 

 this headland it is four feet thick. Thus far it has increased in thick- 

 ness very gradually, and its character has not materially changed, the 

 proportion of pebbles and sand and the size of the former not varying 

 much. At this point however it begins to be rapidly developed, as 

 represented in fig. 4. 



Fig. 4. 



Ochreous flint gravel. 



^^S^=j ^^^ Pebble bed irregularly interstratified with 

 "^^g^^^^g sand. Some of the round flints as large as 

 cannon-balls. 



Whitish and yellow sands. 



The pebbles now become larger ; irregular and false stratification 

 with beds of sand sets in, and in the range of fifty yards this stra- 

 tum attains a thickness of fifteen feet. So rapidly does the change 

 proceed, that by the time we have reached within two miles of 

 Bournemouth, or three miles westward of Hengistbury Head, this 

 conglomerate bed, which at the latter place we have seen to be only 

 three feet thick, is developed to a thickness of about forty feet. This 

 forms, for its limited extent, the most important conglomerate-bed 

 in the English tertiaries. 



Below this bed is a series of laminated and impure sands. In- 

 dications of the variations they present we have already perceived 

 at Barton Cliff and at Hengistbury Head ; there however their de- 

 velopment is confined both as regards thickness and range, but in 

 the Bournemouth cliffs they are exhibited, as we have before said, 

 in perfect continuity for a range of six miles, and with a thickness of 

 probably, as Sir Charles Lyell estimates, about 150 feet. 



The sands are at one spot white, at another yellow, then light red, 

 sometimes coarse, at other places very fine ; the clays are here very 

 carbonaceous and compact — there they are interlaminated with sand ; 

 elsewhere they consist of fine pipeclays ; all these changes are not 

 changes in different parts of a vertical section, but changes in the 

 same bed and on the same levels, with incessant passages from one to 

 the other state of things. In one part of the cliff, as near Boscombe 

 for example, nothing can appear more tranquil and regular than the 

 arrangement of the beds. The upper part of the cliff consists of 

 well-marked, horizontal and uniform, fine white laminated sands, 

 passing downwards into yellow sands, the whole having a ribboned 

 appearance at a distance, and reposing upon horizontally-deposited 

 and laminated fine dark grey clays. 



At a short distance east from this we find these clays almost 

 entirely replaced by sands ; they then reappear again with strongly- 

 marked false stratification. Further still they again disappear, and 

 then they not only reappear as before, but the sands above and below 

 them assume the same lithological characters, and the whole cliff 

 presents a face of laminated clays. And thus it continues from 



