P.G.H. Boswell—Quantitative Methods vn Stratigraphy. 167 
Glacial times, even if its original characters are retained, its detrital 
minerals tell their tale. ‘The pale sands of the Reading Beds, Crag, 
and Glacial beds often resemble one another closely, and it has 
hitherto been difficult or impossible to identify with certainty an 
included mass of such sands in a large glacial disturbance.1 Hxami- 
nation of the mineral assemblage allows a determination to be made, 
for example, as to whether the sand is a pure Reading Sand or one 
contaminated by an admixture of glacial detritus. 
The pebble-bed at the base of the London Clay (on the horizon of 
the Blackheath Beds of the South of England) has in places in East 
Anglia been rearranged upon the shores of the Crag sea. This is 
indicated at times by the iridescent ferruginous coating upon the 
pebbles and the admixture of teeth and coprolites in the bed, but in 
some cases, except for the presence of reddish sand, it is difficult to 
say whether the bed is of Kocene or Crag age (that is, rearranged in 
Red Crag times). An analysis of the sandy material of the bed is 
sufficient to settle the point, for the Pliocene minerals are distinctive 
and easily recognizable, while those of the normal Kocene pebble-bed 
conform to the suite of the Reading Beds. 
In the area where overlap of the Thanet Beds by the Reading 
Beds takes place (in South-West Suffolk and North-West Essex), the 
latter assume the lithological characters of the former and rest 
directly on the Chalk. They contain a ‘bull-head’ flint-bed, and 
are glauconitic clayey sands. When isolated sections in Drift- 
covered outliers occur on the north-west of the main outcrop, it is 
sometimes 2 matter of difficulty, as, for example, at Kedington in the 
Stour Valley, to refer the bed observed to its proper place in the 
geological sequence. ‘The detrital minerals of the sands will usually 
be of service, and in the case quoted it was found that, while the 
Kedington deposit had the lithological characters, etc., of the Thanet 
Beds, its mineral composition showed it to be an outlier of Reading 
Beds, resting on the Chalk, but one which had been rearranged on 
the shore of the Crag sea. Cases might be multiplied, but the 
limited utility of the method must be apparent. 
At present it is premature to discuss the question of the stability 
of minerals in geological time. ‘The controversy upon the extent to 
which andalusite occurs in pre-Pliocene sediments in Western Europe 
is still fresh in our minds. The allied mineral, kyanite, appears to 
have a well-marked earlier limit in the English area, but how far the 
_ presence or absence is due to drainage direction is not certain. The 
extraordinary abundance of sharply crystalline apatite at a loamy 
horizon of the Thanet Beds near Ipswich, protected by a cover of 
Chalky Boulder-clay, is noteworthy as showing how an easily 
decomposable mineral may be preserved. 
The investigation of sediments ‘ hermetically sealed’ shortly after 
their formation, as, for example, gypsiferous deposits in the Permian 
and Trias, may by comparison yield interesting information as to the 
amount of decomposition which takes place in heavy minerals sub- 
sequently to their deposition. 
* The disturbance referred to may be 150 yards long and 40 feet deep. 
