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REV. A. IRVING ON THE PHYSICAL HISTORY OF 



28. The Physical History of the Bagshot Eels of the London Basin. 

 By the Eev. A. Irving, B.Sc, B.A., F.G.S. (Eead January 26, 

 1887.) 



[Abridged.] 

 Introlection. 



To the statement of my views on this subject in the Quarterly Journal 

 of the Society (vol. xli. pp. 492, 507, 508) I adhere in its main 

 outlines, notwithstanding the adverse criticisms upon it which have 

 appeared since in the Quart. Journ. Geol. Soc. vol. xlii. (pp. 402- 

 416). Certainly I felt that, from insufficiency of data, some of the 

 correlations were tentative and open to such corrections as further 

 investigation or criticism might show to be necessary. There can, 

 however, be but little doubt that the method which proceeds by 

 attempting to definitely correlate by stratigraphical evidence the strata 

 seen in the sections on the flanks of the Bagshot district with those of 

 the interior of the main mass involves fewer assumptions than such 

 an attempt as has been made by later authors (Q. J. G. S. loc. cit.) to 

 determine the horizons of those marginal strata by a comparison of 

 them with one another. Underlying the latter process there is the 

 assumption that mere contiguity or proximity to the London Clay is 

 evidence of Lower Bagshot horizons ; but this involves manifestly 

 ' petitio tprincipii? 



I consider that the case must rest mainly on the physical evidence, 

 and that, if no instance remained of a former overlap of lower 

 by higher beds, the case would not be disproved, so long as the 

 stratigraphical evidence was not contradictory ; and the failure to 

 show any overlapping at the present time could be explained by the 

 subsequent denudation of the country, the main lines of which have 

 been, as is. well known, determined far more by the distribution of the 

 plateau-gravels of Quaternaiy or earlier times than by the strati- 

 graphical structure of the district. If later authors, therefore, had 

 disproved the existence of overlap, they would but have disposed of 

 the corollary to the problem, and they would still be confronted by 

 the physical evidence. As regards the paleeontological evidence, it 

 is only right to remark that the very small percentage of the sections 

 in the Upper Bagshot in which fossils have been found, together 

 with the very imperfect state of their preservation (mere irony casts), 

 suggests the possibility that the fossiliferous localities may owe that 

 character to the mere accident of the presence of a little more 

 binding material. Anyhow, the evidence on the affirmative side is 

 too feeble to justify us, on the strength of its mere negation, in re- 

 jecting strata from the higher beds merely on account of absence of 

 fossils, if physical and stratigraphical evidence justifies their assign- 

 ment to such horizons. The fact that " the best-preserved specimens 

 are high up in the series" (Q. J. G. S. vol. xlii. p. 415) may be accepted 

 as a fact strongly corroborative of my vie w as to the marine- estuarine 



