Dr. Irving—Surface- Changes in London Basin. 217 
this for the present, it may be suggested that a due recognition of 
the probability that (in the vast period of time that surface-changes 
have been going.on in the higher parts of the London Basin) 
such materials may have been formed locally, and altogether 
irrespectively of the particular character of the subjacent terrain, 
must make us cautious in attaching very much stratigraphical sig- 
nificance to such observed facts. When we meet with glauconitic 
green sand in a gravel not far from, and resting on beds at a lower 
horizon than, the outcrop of the “‘green-earth series” of the Brackle- 
sham Beds (so-called), there is no difficulty in considering it as 
derived from them; but obviously this is not the case, when ‘‘ green 
sand” occurs in gravels, which overlie Upper Bagshot Sands, as in 
the instance recorded by Mr. Monckton on page 48 of his recent 
paper ‘‘On the Gravels South of the Thames.” 
Among the many interesting observations recorded in the same 
paper, those described on page 43 are of special interest, as illus- 
trating some of the processes of surface-change, accompanied with 
reconstruction of the materials furnished by the Hocene beds and 
the high-level gravels during later geologic times. It is true that 
such “accumulations of sand” in the gravels as are there described 
may by a casual observer be “easily mistaken for the Bagshot Beds 
themselves.” I well recollect the case near Walton Station. Mr, 
Hudleston pointed it out to me on my first visit to the place, when 
the cutting was being widened; and though no gravel was seen at 
that time below it, as it was later on, when the excavations were 
carried further, I gave it as my opinion that the mass was only sand 
included in the gravels; and I felt justified in doing so, not merely 
from the incoherency of the sand and the relation which its rude 
stratification bore to the later contour of the adjacent country, but 
also on the ground that I had a short time before observed the same 
sort of thing at Ascot. In an old note-book I find the following 
note, dated 1885 :— 
S.W. Railway-cutting east of the Race-course Station. “A thin bed 
of pebbles with a few angular flints comes on east of the bridge. 
For some yards this is overlain by a stratified brick-earth. Traced 
to the east, it expands into an ordinary Quaternary drift-gravel 
filling the eroded hollows of the clean sand beneath. Last of the 
second bridge is a sand-pit in the clean white sand of the Lower 
Bagshot. This passes up into a curious contorted mass of fragments 
of a laminated clay mixed with white sand. This in turn passes 
under the gravel.” With a more limited section the loamy bed here 
noted might have been easily mistaken for a genuine Hocene sand- 
bed. The fragments of clay are probably remanié from the basement- 
clays of the Middle Group, which occurred at a somewhat higher 
level. 
In the case of the “somewhat similar deposit in the Brick and 
Tile Company’s brickyard north of Bracknell,” it is not easy to see 
how the section there exposed could have been easily mistaken for 
a Bagshot Bed, as I saw it in 1887. I find the following note of it, 
which I made on the spot:—“ Top of hill, fine section in sand and 
