376 



REV. A.. IRVING ON THE PHYSICAL HISTORY OE 



life-forms as could scarcely happen without some unconformity in 

 the strata. 



It is scarcely necessary to refute an objection to my view on 

 the ground of the considerable interval of time required (Quart. 

 Journ. Geol. Soc. vol. xlii. p. 416), an objection, which, in bringing us 

 down to the Pliocene, seems to be based on a forgetfulness of the 

 Oligocene and Miocene, and of the great space in time represented 

 by the contemporary Nummulitic formation and by the subsequent 

 elevation of some of the loftiest mountain-ranges on the globe. 



The " peculiarity of the ground-plan " referred to is a misconception 

 arising from inattention to altitudes. The northern side of the 

 district, which I have worked more in detail, is found to come out 

 very well on the horizontal ground-plan, as I think will be seen when 

 it is resurveyed. 



Review oe the Physical Evidence. 



1 . Pebbles. — The inference drawn in my last paper (vol. xli. p. 496) 

 was not intended to imply an absence of pebbles in the whole Lower 

 Bagshot ; and I fail to perceive how an inference from a circum- 

 scribed range of data can logically bear such a general interpretation 

 as has been imposed upon it. To the three well-sections there cited 

 may now be added those at Brookwood (Geol. Mag., August 1886), 

 Chobham Place, Bagshot, Dogmersnelcl Park, and Claremont (Mem. 

 Geol. Survey, vol. iv. app.), Thorpe and Hatchfold (recently published 

 by Mr. Whitaker ' Proc. Croydon Micr. and Nat. Hist. Club,' 1886, 

 pp. 54, 66). Neither in these nor in the extensive exposures of the 

 Lower Bagshot on the South-western main line is there a record of 

 them. There is some doubt whether the few "pebbles " met - with 

 in the Ascot Well * were not pyritous nodules. All this does not 

 of course prove a general negative, though it increases the necessity 

 for caution. 



The alleged case recently cited f, with some emphasis, of the occur- 

 rence of pebble-beds in the Lower Bagshot at St. Anne's Hill, Chertsey, 

 breaks down on investigation. The hill is merely an old river-valley 

 escarpment of the Thames, which has undergone the usual process 

 of weathering, with degradation of the higher beds, the debris 

 of which is now strewn on its flanks, partly in the form of talus, as 

 may be well seen in the large pit on the north-east flank of the hill, 

 wherethe pebbles occur roughly interstratifled with green earth}' sand 

 and rotten purplish clay, the crude stratification being just what one 

 is accustomed to in sections of talus-heaps. In places, as at the top 

 of the larger pit, small landslips are detected. The road-section up 

 the flank of the hill shows nothing but such a confused mixture of 

 debris (in this case, sand, clay, and pebbles) as may be generally 

 observed in similar instances. The conglomeratic masses are agglu- 

 tinated portions of the pebble-bed which have fallen from above* 



* Proc. G eol. Aseoc. vol. ix. p. 417. 



t Quart,. Journ. Geol. Soc. vol. xlii. p. 404. 



