Dr. Irving—Surface- Changes in London Basin. 21G 
down pebbles and flint fragments horizontally, but not so with the 
vertical pebbles. A little thought, with the application of the most 
rudimentary mechanical principles, will however reveal the fact 
that the same mechanical law applies in both cases. A pebble free 
to rotate in water upon any axis will assume a permanent direction 
of its longer axis only in the position of least resistance to the 
surrounding medium, and this position is attained, when the 
smallest possible section is opposed to the water. This is equally 
true, whether the water is flowing past a pebble lying on its bed, 
or the pebble is itself moving in obedience to the law of gravitation 
through still water. In the former case the ultimate position of 
the pebble is horizontal, in the latter it is vertical; and if it falls. 
into soft mud or silt it will retain this vertical position after coming 
to rest at the bottom. The conditions, which most readily meet the 
requirements of the problem before us, are those of a detritus- 
laden mass of ice floating and gradually melting away in the water 
of a lake, or the “ backwater” of a broad shallow river. In fact, 
in order to explain the actual phenomena in the one case and in 
the other, we are thrown back upon such conditions as are implied 
in the “ Thames Straits” hypothesis. 
But the facts just described do not stand alone in the immediate. 
locality north of Bracknell, as the following note (made also in 1887) 
of a new section at Messrs. Lawrence’s brickyard close by plainly 
shows :— 
“Upper beds seen in the new pit (about five yards square), a 
somewhat remarkable deposit containing very pure pipe-clay seams. 
(in some cases three or four inches thick), running very irregularly 
through a coarse ferruginous sand, not laminated or distinctly 
stratified. In the sands (generally above the pipe-clay seams) occur 
layers of bog-iron-ore, and portions of the sand are cemented into 
concretionary masses by carbonate of iron. Height of section nine 
feet.” The floor of the open pit was formed by the undisturbed 
pebble-bed. 
This was a new exposure of bed (a) of the published section (F)} 
of my 1887 paper, ‘“‘On the Physical History of the Bagshot Beds. 
of the London Basin,” and the new section strongly impressed me 
with the lacustrine character of the bed. I never saw anything 
like it in undoubted Bagshot Beds. The pipe-clay occurred here 
in a way in which it is occasionally met with in the gravels (as at 
Walton), and was not interstratified (as in the true Hocene beds) 
with regularly bedded sands. I still hold to the view that it records. 
one of the many surface-changes leading to the reconstruction of 
Bagshot materials, supplemented by other material, which would 
vary with the conditions locally prevailing at the time. It is more 
likely than not that the pipe-clay of this and similar deposits, for 
which I have suggested a later date than the Hocene, may be the 
insoluble residue of ice-borne chalky detritus? from the Kennet 
1 Q.J.G.S., vol. xliii. p. 386. 
2 See my paper on “ Physical Studies of our Ancient Estuary,’’? Grou. Mae. for 
1891, Dec. II]. Vol. VIII. p. 357. 
