I OWER CHALK—SOUTH DORS FIT. 
93 
CHATTER VIII. 
THE LOWER CHALK IN SOUTH DORSET 
General Description. 
It lias been stated in the previous chapter that in passing from 
the eastern to the western part of the Isle of Wight the Lower Chalk 
decreases in thickness from over 200 to only 161 feet, and, further, 
that the decrease occurs mainly in the zone of Ammonites varians. 
This thinning of the Lower Chalk continues westward, though not 
quite so rapidly at first, for in the space between Compton Bay 
and Ballard cliff, near Swanage, the decrease only amounts to 
18 feet, the total thickness at the latter place being about 143 feet. 
At Worbarrow Bay, however, eleven miles farther west, it has 
lessened to 92 feet; at Mupe Bay it appears to be 107 feet; at 
Lulworth Cove the beds seen only measure 58 feet, and though 
at this locality there seems to have been some crushing and faulting 
out, yet there is evidence further west that the thickness of the 
Lower Chalk does not exceed 80 feet, which, however, it seems to 
maintain to the vicinity of Abbotsbury. 
It is also a very noticeable fact that this thinning is accompanied 
by, and in all probability is owing to, a diminution in the amount 
of argillaceous matter present. Near Swanage the formation still 
consists of alternating beds of chalk and marl, though the former 
preponderates ; but at Worbarrow Bay this has ceased to be the 
case, and there is nothing to be called marl in the lower part of the 
stage, the only beds of such a nature being in the upper part. 
With respect to the base of the Lower Chalk and the plane of 
separation from the Greensand no mistake can be made. There 
is an abrupt transition from nodular sandstone to sandy chalk, 
and the surface of the sandstone is somewhat waterworn and un¬ 
even, as if it had been swept by currents. The basement bed of 
the Chalk has usually been called Chloritic Marl, simply because 
it contains much glauconite and many phosphatic nodules, and 
occupies a position that corresponds with that of the Chloritic Marl 
in the Isle of Wight. But whether it is strictly the equivalent of 
that bed ( i.e . of the subzone of Stauroneina Carteri), is a 
matter of doubt, for no specimen of that fossil has yet been found 
in Dorset; moreover, casts of Scaphites cequalis are very abundant 
