166 P.G.H. Boswell—Quantitative Methods in Stratigraphy. 
The composition of the different zones of the Coralline, Red, and 
Norwich Crags is similar to, but not of so rich a type as, that of the 
Boxstones. The Chillesford Beds (late Pliocene) are much enriched 
in muscovite. The Westleton Beds (? early Pleistocene) show, as 
compared with the Pliocene beds below and the glacial deposits 
above, rather an impoverished assemblage which may have been 
derived from the Chalk and Kocene beds with a slight admixture of 
Crag detritus (garnets, etc.). The purity of the Westleton pebble- 
beds (mainly flint and quartz) and freedom from foreign boulders is 
interesting in this connexion. The glacial deposits yield, as no 
doubt would be expected when the variety and extent of the rocks 
laid under contribution are considered, a very rich mineral suite, the 
abundance and variety of which are unparalleled by any other deposit, 
and only approached by the Pliocene. Abundant garnets, amphiboles, 
and pyroxenes (sodic varieties included), epidote, micas, apatite, 
staurolite, kyanite, tourmaline, etc., make the deposits easily 
recognizable under the microscope or high-power hand-lens ( X 20). 
It is noteworthy that andalusite is not nearly so abundant as in the 
Crag or in the glacial deposits of the West of England. The post- 
glacial gravels, sands, and brickearths, and the river terrace sands, 
loams, and alluvium, contain similar mineral assemblages to the 
glacial beds, and are no doubt derived largely from them. 
The value and limitations of the application of petrographic 
methods to stratigraphy are well illustrated by the consideration of 
a sequence of deposits in one area such as that just mentioned. 
First we may consider the limitations (apart from those resulting 
from attempted wide correlation). The Upper and Lower Glacial 
beds of East Anglia differ much in field characters, in mechanical 
composition, and in their included boulders, indicating two different 
directions of advance for the ice which produced them. It is very 
difficult, however, to tell the Upper from the Lower beds by the 
heavy detrital mineral assemblage, notwithstanding the fact that 
their mechanical analyses conform to two very different types. This 
may be due to the mineral richness of each. Again, it was hoped 
that the various zones of the Red Crag, when traced northwards, 
might show, together with the increasingly boreal fauna, a corre- 
sponding change in mineral constitution (the basin was closed on the 
south and opened on the north in late Pliocene times). No such 
variation has been observed, and it seems probable that the drainage 
direction and rocks undergoing denudation to produce the Crag sands, 
etc., remained the same throughout Pliocene times. 
On the other hand, the advantages of the method are great. It 
has now become possible to identify any East Anglian bed (allowing 
for the limitations just detailed) from its detrital mineral suite, and 
to say when the mineral assemblage of any particular bed is pure, or 
when it has been contaminated with an admixture of material from 
another bed. A few actual examples may be quoted. Considerable 
glacial disturbance of the East Anglian deposits has frequently taken 
place locally in Southern Suffolk, etc., and it is often desirable to 
_ know whether a limited exposure shows a bed in situ, undisturbed 
and uncontaminated. Should the bed have been redeposited in 
