PEBBLE GRAVELS. 
205 
published views as to the classifieatioQ and order of succession 
in the strata described in Chapters VIII., IX., and X. of this 
work, but brings forward no new evidence as to the relations of 
the different deposits It is, therefore, unnecessary to modify 
tlie classification adopted by the Geological Survey, especially 
as it was founded on a large amount of evidence which was not 
available when Prof. Prestwick’s views were first brought for¬ 
ward. It may also be pointed out that the present Memoir 
contains much corroborative evidence, both palaeontological and 
strati graphical, which has only recently been obtained, through 
the systematic collection and study of the fossils, and through 
the making of trial-borings. 
Some of the outliers referred to by Prof. Prestwich to the 
Westleton Beds have been coloured on the maps of the Geological 
Survey as glacial, and are also considered by Messrs. Wood and 
Harmer to belong to the Glacial series. These will not be 
described here. We will merely draw attention to those outliers 
mapped by the Survey as something differing from, and probably 
older than, the glacial deposits. The common characteristic of 
these outliers is that, unlike the other gravels, they contain a 
large proportion of pebbles of quartz and light-coloured quartzite. 
They usually occur at a considerable height above the sea. 
They overlie Bagshot Sands, and often can be seen to pass under 
the Chalky Boulder Clay, but their stratigraphic! 1 position 
cannot be more exactly fixed. 
One of these outliers has been mapped near Hertford, by 
Mr. Whitaker, who speaks of a “small deposit of pebble-gravel 
that occurs on the London Clay hills between Hertford and 
Hoddesdon, and which may be of pre-glacial age. It is composed 
almost wholly of pebbles, the larger mostly of flint, and the 
smaller of quartz, and it has been inferred to be the oldest 
Drift-bed in the district (and to the west), and to have been 
largely denuded before the deposition of the succeeding bed.”* 
From Hertford these gravels extend in a south-westerly 
direction, capping all the higher hills as far as Rickmansworth. 
After an interval of a few miles they reappear on high ground 
near Araershain, and again, on the south side of the Thames, 
not far from Marlow. To the east small outliers are found on 
Hampstead Heath, at a height of about 400 feet above the sea. 
At High Beech the gravels reappear, and again on the highest 
points in the county, at Havering, Brentwood, Langdon Hill, 
and Hadleigh. Crossing the Thames, we find an outlier on 
Shooter’s Hill and Swanscomb, which may be of the same age. 
As full details of all these outliers are given in a recently 
published Memoir,f it will be unnecessary to redescribe them 
here. 
* Geology of the N.W. part of Essex and the N.E. part of Herts, &c. {Mem. 
GeoL Survey), p. 32. (1878.) 
t Geology of London, vol. 1 {Mem. Geol. Survey)^ pp. 290-296. (1889.) 
