356 Rev. A. Irving — The Broohoood Well-Section. 



calcareous material seems to have been entirely dissolved away, and 

 their surfaces are coated with a layer of coarse sand-grains, as if 

 they had rolled about in sand when softened at their surfaces by 

 water. They present a very different appearance to that presented 

 by the specimens found in situ in the London Clay. These Bagshot 

 cement-stones may therefore be considered as derived in all proba- 

 bility from the London Clay itself. 



5. The least thickness we can assign to the London Clay in 

 this section is 371 feet, not including the passage beds at the 

 top. In the paper referred to above 1 I have drawn attention 

 to the greater thickness of the London Clay in these deep-well 

 sections of the more central portions of the main mass of the 

 Bagshot Sands, as compared with the thickness of the London Clay 

 beneath the Bagshot Sands, nearer the north and south margins (e.g. 

 in the Aldershot Town Well and the Wokingham Well). This 

 difference of course might arise either (1) from attenuation of the 

 London Clay owing to the conditions of original deposition, or (2) 

 to destruction of the upper beds of the London Clay by denudation 

 during the earlier and middle portions of the Bagshot period. The 

 strong evidence we have- now in at least two of the more central 

 well-sections of a passage from the London Clay into the Bagshot 

 beds above, goes to show that deposition was continuous in the 

 central portions of the area ; while the abrupt transition from the 

 one formation to the other and the frequent erosion of the London 

 Clay surface beneath Bagshot Beds observable in sections on the 

 northern and southern margins point to the obvious conclusion that 

 the London Clay was there at the same time undergoing destruction 

 by denuding agencies. If this be admitted, it is hardly necessai'y to 

 point the inference that a slight synclinal curvature was given to the 

 London Clay during the earlier portions of the Bagshot period. But 

 this is not all. This Brookwood. section is the first, so far as I am 

 aware, to throw some light upon the horizons occupied by the 270 

 feet of London Clay penetrated in the deep well at Wokingham. A 

 reference to the table of strata given by Prof. Bupert Jones, F.B.S., 

 for the Wokingham Well, 1 and comparison of it with the London 

 Clay strata penetrated in this Brookwood section, brings out a remark- 

 able correspondence (in the occurrence of layers of septaria, of 

 pebbles, and of pyrites) between the 273 feet of London Clay at 

 Wokingham and the lower 200 feet or rather more of the London 

 Clay in the Brookwood section ; while from the upper 150 feet or so 

 of the London Clay in the latter section there is no record of such 

 occurrences. So close is the correspondence here cited that there is 

 only a difference of a foot or so in the distance of one of the layers 

 of pebbles in the two sections above the base of the London Clay. 

 These observations harmonize too with the well-known fact that the 

 London Clay becomes more arenaceous, and has a less pronounced 

 marine character in its upper beds, where shown in its full normal 

 development. 2 Of the two alternatives suggested above, we are 

 surely justified, in the light of this new evidence, in preferring the 

 denudation to the original attenuation explanation of the facts, and 

 1 Q.J.G.S. toe. cit. 2 Cf. Etheridge, he. cit. 



