BAGSHOT BEDS OF BAG SHOT HEATH. 51 



Middle Bagshot, more especially as we find very similar unfossi- 

 liferous clays between the fossiliferous green sand-beds in the 

 corresponding part of the Bracklesham Series at Whitecliff Bay in 

 the Isle of Wight. With reference to Dr. Irving's proposal to 

 include the Middle Bagshot of Prof. Prestwich in his ' Lower 

 Freshwater Series,' I would point to the abundance of marine fossils 

 in the green sand-bed at Ascot and Goldsworthy, and other places, 

 nor does it seem to me that his reference to shells thrown on to new 

 sand-banks by wind in the Rhone Delta ^ is really relevant to the 

 point at issue. The casts of the large Venericardia planicosta and 

 Corhula gallica are abundant in the Ascot cutting with valves 

 closely shut, and therefore the probability is that they lived where 

 we now find them. 



Though Dr. Irving speaks of the fossil shells being broken and 

 worn, he does not prove that they were so broken or worn before 

 they were embedded, and I possess a cast of Corhula gallica from 

 Ascot which tells the following history : — The shell was buried in 

 the sand with its valves shut tight and unbroken. The shell was 

 crushed and broken into fragments and afterwards destroyed ; now 

 the cast alone remains. Had the shell remained and not the cast it 

 would have been in fragments, and Dr. Irving might have then called 

 it broken and worn. 



It may be added that the authorities of the Museum of Practical 

 Geology, Jermyn St., have accepted the best part of my collection 

 of Upper Bagshot fossils and also a set from the Middle Bagshot 

 collected by Mr. Herries and myself. 



The brick-field just described furnishes to my mind a very satis- 

 factory answer to the argument used by Dr. Irving to prove the 

 thinning out of the bed of green sand to the north of Wellington 

 College. The argument is this : — The green sands, or ' green-earths' 

 as he calls them, diminish in thickness from 36 feet in a well at 

 Ambarrow near Wellington College, and 41 feet in the Wellington 

 College well, to 18 feet one mile north of the former. Therefore, if 

 we restore them northwards with a proportionate rate of attenuation, 

 they will thin out altogether.^ I do not know whether Dr. Irving 

 would adopt the similar calculation that as the earths above alluded 

 to are 36 feet thick at Ambarrow and have thickened to 41 feet at 

 Wellington College 5 furlongs to the north-east, tberefore at Bill 

 Hill, Bracknell, 35 furlongs to the north-east, they will be 71 feet 

 thick. Probably he would not, nor should I. In the first place, I 

 must be allowed to doubt whether the 41 feet of green sand at 

 Wellington College were wholly green sand, and the 36 feet at 

 Ambarrow are, according to Dr. Irving's own section, largely made 

 up of grey sand and clay ; and secondly, looking at the irregularitj- 

 in the shape and occurrence of beds of green sand ut Ascot and 

 other places, an argument such as Dr. Irving uses is hardly 

 applicable to strata so variable. 



1 Geol. Mag. for 1891, p. 361. 



2 See ' Recent Contributions to the Stratigrapby of the later Eocenes of the 

 London Basin,' by Dr, A. Irving, Wellington College, 1891, p. 7. 



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