P.G.H. Boswell—Quantitative Methods in Stratigraphy. 165 
a wide area, all the Eocene beds in the London Basin appear to 
conform more or less to a general type characterized by the occurrence 
of abundant kyanite, staurolite, tourmaline, zircon, and rutile, and 
less commonly, green hornblende and small colourless garnets. The 
various beds of the Pliocene Series from Cornwall to East Anglia and 
Belgium all possess more or less similar mineral assemblages, the 
chief members of which are red garnet, muscovite, andalusite, 
staurolite, epidote, etc. The distinction from the Eocene Series is 
very marked. 
As a final example may be quoted the sands forming the lowest 
part of the Inferior Oolite, the mineral composition of which is very 
distinctive, and yet maintains practically the same character when 
the beds are traced from Yorkshire by way of Northampton, the 
Cotteswolds, Bath, and Yeovil to the Dorset coast. 
While correlation of smaller geological divisions over considerable 
areas is fraught with difficulty, the mineral constitution of the 
different beds is of great stratigraphical value, as has been stated, 
over limited areas. It has been shown, for example, that the Thanet 
Beds, Reading Beds, and London Clay in East Anglia have each 
a characteristic mineral assemblage by which they may be recognized, 
provided that we know also the mineral composition of all the other 
Tertiary beds from the top of the Chalk to the post-Glacial and 
Recent.! Speaking generally, each mineral assemblage remains 
constant over an area of nearly 500 square miles, and appears to be 
independent of the lithological variations of the bed containing it.’ 
That there is a limit to this constancy is shown by the fact that the 
Reading Beds, when traced as far to the south-west as Bishop’s Stortford 
and Hertford, begin to show a variation in mineral character. The 
change in mineral composition of the Reading Beds with respect to 
the underlying Thanet Beds may be due in part to difference of 
mechanical composition, and therefore to difference in conditions 
of deposition, resulting in concentration of certain minerals and 
decomposition of others, but whatever may be the cause the variation 
observed has an important stratigraphical value. The green and 
brown hornblende, pyroxene, biotite, apatite, etc., of the Thanet 
Beds disappear, and it is not until we reach the London Clay that 
some of these minerals reappear. The large and characteristic grains 
of kyanite, staurolite, and tourmaline present in the Reading Beds 
have disappeared in the London Clay, but green hornblende becomes 
_ abundant, and muscovite and colourless garnets more common. ‘The 
detrital material is of fine grain, and therefore provides the more 
contrast with the very coarse stuff found in the Boxstone Bed at the 
base of the Suffolk Crags. 
1 Abstr. Proc. Geol. Soc., No. 973, p. 76, 1915. 
2 Very recently (March, 1916), in a lecture before the Geologists’ Association, 
my friend and colleague, Mr. V. C. Illing, has claimed that the horizons of the 
unfossiliferous and oil-bearing sediments of the south-western part of Trinidad 
may be identified and correlated over a limited area by means of their mineral] 
assemblages. If this contention is supported by the evidence (and from an 
examination of the residues I believe it is), the hitherto purely academic study of » 
the petrology of sediments becomes at one bound of great economic importance. 
