350 
THE CRETACEOUS ROCKS OF BRITAIN. 
is more marly and less chalky over the whole of the east of England 
as far north as Mildenhall, in Suffolk, and westward as far as 
Berkshire, Hampshire, and the Isle of Wight. Beyond this area 
both to the west and to the north argillaceous matter rapidly dis¬ 
appears, and the material, which is still often called Chalk Marl, 
becomes a chalk with only a few included bands of marl. The lateral 
change from chalk marl to chalk in Dorset has been described on 
p. 107, and the similar change in Norfolk is mentioned on p. 209. 
Hence it would appeal* that the muddy sediment came from the 
same source as the Gault clay—namely, from the south-east. 
The conditions in the south-west of England are especially worthy 
of attention. In the first place, the purity of the chalk and the 
-small thickness which can be regarded as representing the Chalk 
Marl (zone of Am. varians) in southern and western Dorset seem 
to indicate clearness of water and absence of terrigenous sediment; 
but it is curious to find that this clear water was tenanted by a small 
amount of life. 
It is still more curious to find that this area of pure chalk ex¬ 
tending into Devon as far as Membury, while less than five miles 
further west the Chalk Marl is represented by a calcareous sand¬ 
stone consisting mainly of large quartz grains. It is stated on p. 126 
that this sandy zone of Ammonites Mantelli probably represents 
that part of the Chalk Marl which is absent in West Dorset; but if 
this is so, and if the Membury and Chard chalk is wholly or largely 
of later date than the zone of Ammonites Mantelli at Wilmington and 
Beer Head, how did it come to pass that the sandy beds are absent 
below the chalk, and that the chalk is absent above the sands. 
The only explanation of these facts which has suggested itself 
to me is that the sands of the Ammonites Mantelli zone were origi¬ 
nally deposited over a much wider area than that to which they are 
now restricted, that they extended far both to the northward and 
to the eastward, not only over the Mem aburynd Chard district, 
but far into the centre of Dorset. The very coarseness of the sands 
indicates the existence of a strong current, and we have only to 
suppose that this current became still stronger till it removed a 
large portion of the sand which had been accumulated ; then it 
gradually slackened as further subsidence took place and the nodule 
bed which lies at the base of the chalk in West Dorset and Somerset 
was formed on the current-swept surface. 
Those portions of the zone of Ammonites Mantelli which are 
preserved testify to accumulation under the influence of changing 
currents by the rapid variation in the thickness of the component 
beds within short distances (see Eig.31, p. 131), and by the surfaces 
of erosion which often occur between them. Their preservation 
may be attributed to one or other of two causes or to both com¬ 
bined ; either they had become more consolidated than the portions 
which were destroyed or they lay out of the track of the strongest 
part of the current. The latter supposition will also serve to explain 
