Correspondence — Mr. S. J. B. Skertchley. 335 



taken by many penologists to the employment of the term andesite. 

 The lavas of Cant Hill were also probably of an andesitic character, 

 so that, so far as original mineral constitution is concerned, there is 

 some apparent justification for the mapping of both of these rocks 

 as " greenstone " by the Geological Survey. 



3. "The Bagshot Beds of the London Basin." By H. W. 

 Monckton, Esq., F.G.S., and R. S. Herries, Esq., B.A., F.G.S. 



The authors stated that their object was to describe more fully the 

 Lower Bagshot beds, and to disprove the view lately advanced by 

 Mr. Irving that, in certain places, the Upper Bagshots overlap the 

 Lower and rest directly on the London Clay. They described or 

 referred to a number of sections all round the main mass, beginning 

 at St. Ann's Hill, Chertsey, where they considered that the mass of 

 pebbles and associated greensands must be referred to the Middle 

 Bagshot. The outliers near Bracknell and Wokingham were shown 

 to consist of Lower and not Middle Bagshot, which does not appear 

 in the valley north of Wellington College. 



The Aldershot district was explained, and it was shown that the 

 beds there resting on the London Clay were Lower and not Middle 

 Bagshot, and the occurrence of fossils in the Upper Bagshot of that 

 district was recorded. 



The conclusions that the authors came to were, that a well-marked 

 pebble-bed was almost always present, marking the division between 

 the Upper and Middle Bagshots, but that there were other pebble- 

 beds of a less persistent character occurring both in the Middle and 

 Lower Bagshot ; that the Lower Bagshots generally consist of false- 

 bedded sands with clay laminse and no fossils except wood, whereas 

 the Upper Bagshots are rarely false-bedded, and are characterized 

 by the absence of clay bands and the presence of marine fossils ; and 

 that the Middle Bagshot is a well-marked series consisting of green 

 sands and clays. 



They claimed, in conclusion, that there was no reason for dis- 

 turbing the old reading of the district, and that there was no evidence 

 of an overlap of the Lower Bagshots by the Upper. 



COEEESPOUIDE1JCE. 



SLICKENSIDED SURFACES OF CHALK. 



Sir, — Mr. H. Hutchins French, F.G.S., and I have discovered 

 widely spread surfaces of chalk slickensided horizontally in the 

 neighbourhood of Sutton, Surrey. There seems to be abundant 

 evidence that these markings are really due to friction, and they are 

 associated with a remarkable cleaved structure at a high angle to 

 the bedding. This cleavage is very striking, and we should be 

 glad to receive any notices of similar phenomena in the Chalk. 



We also find the Thanet Sand to extend further west than was 

 supposed to be the case, it being some 15 feet at Leatherhead, where 



