HERTFORDSHIRE NATURAL HISTORY SOCIETY. 
xliii 
by the infiltration of silica. At Watford this shingle-bed is 15 
feet thick, the black, rolled flint-pebbles being in a matrix of 
ochreons clayey sand, but not consolidated; and in fact it is only 
at this spot that the shingle-bed can now be seen in situ in the 
form of a conglomerate, or, as it is sometimes called, “plum-pudding 
stone.” Blocks of this conglomerate occur, however, here and 
there, over a considerable portion of Hertfordshire, and they have 
also been seen in Essex. We know that the Beading beds once 
extended over the greater portion of Hertfordshire, by the outliers 
which are seen at various places, and it seems more probable that 
the blocks we meet with are relics of these beds of which the looser 
portions have been carried away by denudation, than that they have 
been transported in a northerly direction from the neighbourhood 
of Badlett. It is probable that the land over which flowed the 
rivers which deposited the clays, sands, and pebbles of the Woolwich 
and Beading Series was to the south of the London and Hampshire 
Tertiary Basin, where is now the English Channel, and on this 
supposition we should expect to see the beds to the north more 
marine in character than the more southerly portion of the series. 
Erom the very variable character of the whole of this series it may 
be inferred that in no locality where it occurs was it, or any portion 
of it, a deep-sea deposit. Littoral conditions may have prevailed 
over at least a considerable portion of Hertfordshire, so that there 
is no improbability in the shingle-bed having at one time been of 
considerable extent, and throughout this extent it may anywhere 
have been consolidated into a conglomerate. 
We now, therefore, see before us a fragment of an old sea-shore 
which is usually seen as a bed of loose flint-pebbles in sand, some¬ 
times clayey, the pebbles having been derived from the denudation 
of the Chalk. How is it then that it occurs here in the form of a 
conglomerate, not, as is most usual, with a matrix solidified by 
calcareous matter, but by siliceous ? Professor Prestwich, in his 
well-known paper on “ The Woolwich and Beading Series,” * says 
that in 1846 the mottled clays of the “ Argile Plastique” of the 
Paris basin were found to contain “ a very considerable proportion 
of gelatiuous or soluble silica, i.e. silica in an active chemical state, 
and soluble in alkaline solutions without fusion.” He then says 
that he has “ recently tested the mottled clays from various 
places in the London district,” and has found that “ they also 
contain this gelatinous silica, which can be readily separated by 
boiling in a solution of caustic potash,” the proportion being, how¬ 
ever, very variable. As the stratum which immediately overlies 
our pebble-bed consists in this neighbourhood of mottled clays, it is 
probably the gelatinous silica from them which has converted the 
bed into a conglomerate, and which, in other places, where sands 
underlie these clays, has converted the sand here and there into a 
sandstone. As blocks of our conglomerate are found scattered far 
and wide, so are blocks of this sandstone, which are known as 
“ sarsen-stones, ” “ grey-wethers,” and “Druid sandstones,” and 
* ‘ Quart. Journ. Geol. Soc.,’ vol. x, p. 75. 
