450 Sir H. II. Howorth— 



its colour and superficial appearance. These peculiarities, which 

 mark it over a wide area from Yorkshire to Finohley, and from 

 Southwold to Warwickshire, are, nevertheless, a secondary, and not 

 a primary, feature of the clay, and have disguised and confused the 

 problem of its explanation. 



It has been noticed by several writers that, while there is a 

 common appearance to this clay wherever found, due to the fact 

 that it contains much debris of chalk strata, yet that in regard 

 to its other contents, and notably its matrix, it varies in accordance 

 with the composition of the beds over which it lies, that is, with the 

 substratum. This fact has been frequently noticed, and was, so far 

 as I know, first observed by the Rev. W. B. Clarke, who, writing as 

 far back as 1837, says: — "The diluvial clay covers a great portion 

 of Suffolk, Norfolk, Cambridgeshire, and Essex, and at Cromer rises 

 to 400 feet ; much of it is yellowish, but the greater part blue. 

 In both cases it contains chalk pebbles, sometimes in layers, but 

 generally dispersed. This at once distinguishes it from the London 

 and Plastic Clays." Mr. Clarke then goes on to argue that " the 

 yellow clay was derived from the plastic, and the blue, from its 

 peculiar fossils, from the clay below the Chalk " (Geol. Transactions, 

 ser. ii, v, p. 365). 



These observations of a very good geologist have been amply 

 confirmed by later explorers. Thus Mr. Skertchly says, speaking 

 of the Chalk, the Kimeridge and Oxford Clays: — "We find that 

 the Boulder-clay lying upon these rocks partakes of their physical 

 character. Thus, upon the Chalk the Boulder-clay is very chalky, 

 and, indeed, in some places, as at Mareham le Fen in Lincolnshire, 

 and Thetford in Norfolk, it is almost entirely made up of that 

 substance ; at the former place it is quarried and burnt for lime, 

 and at the latter the presence of seams of clay and ice-scratched 

 flints alone enables us to discriminate between it and the chalk 

 beneath. The Kimmeridge Clay is darker than the Oxford Clay, 

 and we accordingly find the Boulder-clay which reposes upon the 

 former is darker than that which lies upon the latter. Where boulders 

 are rare, it is sometimes very difficult to distinguish the Boulder- 

 clay from the older rocks." (" Great Ice Age," new ed., p. 346.) 

 " The Gault clay again takes the ground in but a small area 

 in the Fens, but the Boulder-clay ' picks it out ' as it were, 

 and at Modney Bridge brickyard, near Hilgay, for example, I have 

 known the glacial bed to be mistaken for Gault by persons quite 

 familiar with the latter." "Similar remarks," says Mr. Skertchly, 

 " apply to all other formations upon which I have mapped Boulder- 

 clay. For example, the light-blue Upper Lias Clay of Leicestershire 

 impresses its character upon the Boulder-cla} 7 which overlies it, and 

 the other members of the Liassic group, where they are in force, 

 behave in a similar manner. The Great Lincolnshire (Inferior) Oolite 

 Limestone around Melton Mowbray yields so large a quantity of 

 material to the Boulder-clay there, that I have been in doubt as 

 to whether the deposit might not be faulted limestone. These 

 peculiarities are at once and correctly expressed by the statement 





