Reviews—Whitaker’s Geology of London. 571 
zone of the Lower Chalk is termed the zone of Ammoniies varians. 
Without the aid of these rock-beds but little progress could have 
been made in mapping the several divisions of the Chalk, for fossils 
in this, as in other formations, are not always to be found when 
most wanted. 
The London Clay has not furnished much material for the believer 
in definite zones, for the fossils are alike through the formation, 
although some species are more abundant at certain horizons. Over 
large areas the London Clay appears to be practically barren of 
organic remains, indeed Mr. Whitaker mentions that he spent some 
days in examining new railway-cuttings in the London Clay in 
Essex, without meeting with any palzontological reward. 
In dealing with the Hocene strata, Mr. Whitaker speaks of the 
practical importance of separating deposits that can be mapped 
distinctly even if there are no great paleontological distinctions in 
their fossils. This is quite right if we wish to interpret properly 
the rocky structure of a country, and show the relations of the strata 
ta the form of the ground. The general remarks on the Lower 
London Tertiaries, and the full account given of the History and 
Jiiterature of the subject, are for the most part new. Herein the 
author criticizes recent observations and views of Mr. Starkie 
Gardner, Mr. G. F. Harris, and others. 
The Drifts that irregularly overlie the Bagshot Beds and older 
strata have given much trouble in determining their respective ages. 
But the process of minute correlation, which to some minds appears 
an essential basis for geological happiness, cannot always be carried 
out; and would lead to much unprofitable anxiety in the matter of 
Drifts. There may be no paleontological evidence, lithological 
characters may be of no avail, and; more serious still, there may be 
no stratigraphical evidence. Hence we can well understand the 
reason for a chapter on “Deposits of Doubtful Age.” In some 
instances where outlying patches of gravel occur on London Clay, 
it is impossible to determine whether or not they are of Pliocene 
(‘‘pre-Glacial ”) age, whether they are Glacial gravels newer or 
older than the Boulder-clay, or whether they may have been 
derived in comparatively recent times (during the denudation of 
the country) from any one of these accumulations, or from the 
pebble-beds of Bagshot age. 
The broad general distinction in the gravels seems to be this. 
The pebble-beds of Hocene age are almost entirely made up of flint. 
The “ pre-Glacial ” (and possibly Pliocene) gravels are made up of 
flint and quartz pebbles. The Glacial gravels contain in addition 
many pebbles of quartzite, and sometimes derived Jurassic fossils : 
and they are rudely stratified and sometimes contorted. The River 
gravels contain all ingredients and are more distinctly stratified. 
But in Drift deposits there are many exceptions to every rule, and 
much of the Glacial gravel of Finchley is indistinguishable from the 
presumably older “ Pebbly Gravel” of the Geological Survey. 
The Clay-with-flints is also one of the accumulations of Doubtful 
Age, a residue left during many ages and forming now, by the slow 
decomposition of Chalk-with-flints by atmospheric actions, leaving 
