BAGSHOT STRATA FROM ALDERSHOT TO WOKINGHAM. 509 



easily-^follow from occasional and local intrusions of the sea (perhaps 

 at unusually high tides), owing to the shifting nature of the land- 

 barriers which are formed and removed from time to time at the 

 sea-margin of a delta. 



Discussion. 



Prof. Prestwich, while admitting some of the facts recorded 

 in the paper, could not agree with the Author in some of his con- 

 clusions drawn from those facts. The variations in thickness of the 

 London Clay were no proof of actual unconformity. His many deep- 

 well sections were, he thought, of much interest. At the time he had 

 himself studied the country, the district was a wilderness, and there 

 was only one well-section into the Chalk in the district. He did 

 not attach the same importance to the minute subdivision of the 

 Bagshot beds which the Author did. He did not think sufficient 

 allowance had been made for local variations in the divisions of 

 such beds as the Bagshot series. 



Prof. Efpert Jones thought, with the Author, that the 

 pebble-beds marked the lapse of considerable intervals of time. 

 He thought that the variations in the thickness of the London Clay 

 were very interesting ; but the persistence of the Woolwich Beds 

 showed that the flexure of the strata took place after the deposition 

 of the Bagshots. He had seen an apparent unconformity between 

 the Bagshots and the London Clay at Bracknell. 



Mr. Whitaker stated that when the geological map of the dis- 

 trict was made by his colleagues very few sections existed. He 

 thought that the pebble-beds were of importance, though not 

 persistent. Mr. Polwhele trusted rather to the green bed than to 

 the pebble-beds as his guide in mapping the Middle Bagshot. The 

 occurrence of pebble-beds composed entirely of Chalk-flints was a 

 proof of the existence of an unconformity somewhere, as they 

 showed that the sea of the period must have reached the Chalk. He 

 had pointed out the likelihood of this having occurred in the far 

 west of the London Basin. 



Mr. MoNCKTON entirely disagreed with the interpretation of the 

 sections by the Author. He regarded well-sections with the very 

 greatest suspicion. He considered the green sand the type of the 

 Middle Bagshot. He agreed with Mr. Whitaker that the Author 

 had confused pebble-beds on dijfferent horizons, some near the top 

 of the Middle and others in the Lower Bagshots. He thought the 

 sands and pebble-beds of the Lower Bagshot were always clearly 

 distinguishable from those of the Upper Bagshot. 



The Author admitted that pebble-beds alone did not define 

 horizons, and he had not referred to them in this way except where 

 the sequence of beds above and below them was undoubtedly clear. 

 He had endeavoured in the paper to explain the remarkable persist- 

 ence of that which occurs at the base of the Upper Bagshot sands, 

 a J. G. S. No. 163. 2 N 



