406 8. A. Adamson — Fossil Tree near Bradford. 



difficult to see how we can arrive at any other conclusion than one 

 favourable to unconformity, from evidence furnished by the London 

 Clay itself. 



Where this formation exists in its full normal development, with 

 Bagshot Beds superimposed upon it, thei-e appear to be clear signs 

 of a passage from one formation into the other : on the other hand, 

 where — as in sections elsewhere described by me — on the marginal 

 portions of the Bagshot area, the transition is an abrupt one, the 

 stratigraphical data furnished by the London Clay seem to lead to 

 the necessary inference, that the upper beds of that formation, to the 

 extent of probably 100 to 150 feet in thickness, were removed by 

 denuding agencies (furnishing perhaps the clayey materials of the 

 Middle Bagshots 1 ) before the Bagshot Beds were deposited upon the 

 London Clay in those localities. 



Note. — In generalizing from a number of deep well-sections in a 

 given geological district, it would be mere pedantry to insist upon 

 the exact value of the measurements in every case. Errors may 

 arise from tubes getting out of the vertical, and from the materials 

 getting mixed up to some extent in the boring tools. But they no 

 more vitiate the conclusions drawn from a general agreement of a 

 number of sections than those slight variations allowed for on the 

 score of "errors of experiment" in experimental science invalidate 

 the conclusions drawn from a general agreement of results. In some 

 instances, moreover, we have more definite data than a mere boring 

 can furnish. At Brook wood, for example, the six-foot well with 

 iron-cylinders was carried down to a depth of 197 feet, that is to 

 say, through the whole of the Bagshot series and into the London 

 Clay; at Wellington College the six-foot well penetrates about 

 180 feet; in the Ascot deep-well mentioned above the shaft or well 

 extends down to 250 feet ; and in the deep well at Wokingham the 

 six-foot "dry-sinking" was carried down to 264 feet, that is, into 

 the Basement-becl of the London Clay. 



IV. — Notes on the Discovery of the base of a Large Fossil 

 Tree at Clayton. 

 By S. A. Adamson, F.G.S. 

 iNLY a short time since there was discovered in the Lower Coal- 

 measures at Idle, a magnificent specimen of Megalichthys 

 Eibberti; and now, at Clayton, near Bradford, has been found one of 

 the grandest examples yet seen of a fossil Sigillaria tree. It was in 

 the Fall Top Quarry, at Clayton, worked by Messrs. Murgatroyd and 

 Sons, that this remarkable fossil was discovered, and these gentlemen 

 deserve the highest praise from all geologists for the skill and 

 extreme care with which they have bared the fossil, and also for 

 their kindness in allowing it to be inspected. This quarry is not far 

 from the edge of a bold escarpment overlooking the Thornton 

 Valley, and the well-known Ellancl Flagstone is worked hero for 

 landings, flags, etc. Between the Better-Bed-Coal and the Flagstone 

 1 Cf. Proc. Geol. Association, vol. ix. No. 5. 



