212 Dr. Irving—Surface- Changes in London Basin. 
paper (supplementary thereto) which appeared in the GnronoGgicaL 
Magazine ' for September, 1890. In this last paper the hypothesis 
of a terrestrial and fluviatile origin is extended even to the high- 
level inland gravels of the “‘Mundesley and Westleton” series of 
Prof. Prestwich. 
(c) In 1891,? I put forward the same view even more explicitly 
in the published summary of a lecture delivered at Windsor on 
“The Geological History of the Thames Valley.” There it is 
pointed out how the maximum elevation of the Wealden axis in 
Pliocene time, and the correlative “ tilting tothe north of the Hocene 
strata of the Tamisian area,” probably furnished all the conditions 
required for the accumulations of the plateau-gravels, and initiated 
the present Valley of the Thames itself; and a comparison is drawn 
between this supposed Pliocene set of conditions and those which 
can be observed at the present day, where the Hastern Alpine rivers 
spread their sheets of detritus over the Tertiary terrain of Bavaria 
and Wiirtemberg, owing to the northerly tilting given to it in the 
last great stage of Alpine elevation. It was from observations of 
these in my travels of former years, that the terrestrial origin of 
our plateau-gravels in Berks and Surrey first clearly suggested 
itself to me. 
In this northerly tilting of the London Tertiaries is to be found the 
explanation of what must otherwise appear an anomaly, in the fact 
that in my 1890 paper I have been able to recognise gravels of the 
platean-series down to levels as low as 300’ and even 200’ above O.D. 
(Bearwood and Farley Hill), while further south I have not been 
able to bring gravels at a rather higher altitude (with the present 
geotectonic structure of the country) into the category of the 
plateau-gravels, as for instance those on Hartford Bridge Flats. 
To draw the line between the original plateau-gravels and the 
secondary gravels of the district, which are either wholly or in part 
derived from the widely-scattered material of the ancient plateau, 
is no easy matter, for the two seem to merge into one another, 
Relative altitudes will not settle the matter, for a gravel, which is 
nothing more than a mass of material reconstructed out of an ancient 
vravel of the true plateau-series, may be actually at a higher level 
than one of that series farther north, owing to the general lowering 
of the gradient of the northward sloping plateau, from the pro- 
gressive rate of denudation as you ascend from lower to higher 
altitudes. After some thought on the matter, and trying first one 
then another test, by which to differentiate the plateau-gravels from 
the secondary or terrace-gravels (in which category I have included 
not only the “glacial gravels” but all the “patches of unstratified 
and angular gravel found on the higher slopes and minor bluffs 
of the present valleys, mere ‘run-of-the-hill’ of the later denudation 
of the country”), I was unable to find any other basis of classification 
than that which combines both structure and composition. And if 
1 “Note on the Elevation of the Weald,’’ Grot. Mac. Dec. III. Vol. VII. 
p. 403, e¢ seq. 
? See Science Gossip for May and June, 1891. 
