2 THE CRETACEOUS ROCKS OF BRITAIN. 
A few words may also be said respecting the original extension of 
tlie Chalk over areas from which it has since been removed by detri- 
tive agencies. That it was originally continuous over the elongate 
dome of the Wealden-Boulonnais area is a generally accepted fact, 
and that the formation also extended very far westward beyond its 
present boundary line will likewise be admitted by everyone, but we 
have no means of knowing how far it actually spread in that direc¬ 
tion. There are gravels containing Chalk flints near Torrington in the 
north-west of Devonshire and also in South Wales (G-lamorganshire) 
up to heights of 400 feet, and though these may not have been directly 
derived from the Chalk, it is highly probable that Chalk once covered 
large parts of those districts, and extended beyond the mouth of the 
Bristol Channel. 
It can hardly be doubted that the Chalk was deposited over the 
whole of Central England, and extended across Cheshire and the 
Irish Sea to Antrim, where large tracts of it still exist beneath the 
protecting cover of the Lower Tertiary basaltic lavas. 
That the Chalk, and especially its middle and upper portion, must 
have had a great westerly extension may be inferred from the great 
purity of the Chalk in the outliers which still remain in Devonshire, 
and from the great thickness of the formation as a whole. In the Isle 
of Wight its thickness is not less than 1,600 feet, in Norfolk it is prob¬ 
ably about 1,300 feet, and in Yorkshire Mr. Lamplugh has estimated 
it at 1,270 feet, though the highest beds are not there exposed. 
The continuity and comparative uniformity of the material com¬ 
posing this great Chalk formation have been a considerable hindrance 
to its scientific exploration. It is really divisible into at least three 
stages, each of which has special lithological characters and a special 
assemblage of fossils, but it is only within the last twenty }^ears that 
these stages have been adequately recognised in England. 
From this point of view the Chalk contrasts strongly with the 
Jurassic Series, which is obviously divisible into a number of con¬ 
secutive rock-groups, each with a different set of fossils. Conse¬ 
quently when William Smith began to record the sequence of rocks 
in the neighbourhood of Bath he readily recognised the several local 
members of the Oolitic Series, as well as the overlying beds of “ Blue 
marl ” (Gault) and Greensand, but the Chalk appeared to him as a 
single formation, incapable of being definitely subdivided, except into 
a lower part without flints and an upper part with flints. 
One of William Smith’s contemporaries, however, whose residence 
in the Yale of Pewsey afforded him excellent opportunities of obser¬ 
vation, divided the Chalk into three parts, and he was, in fact, 
the first to describe the bed which is now known as the Chalk 
Rock. This observer was the Rev. J. Townsend, whose book, 
published in 1813, is a mine of curious and generally accurate infor¬ 
mation hidden under a title which is not calculated to attract the 
geological reader. * 
* The Character of Moses established for Veracity as an Historian, by 
Rev. J. Townsend. 4to. London and Bath, 1813. 
