By A. J. Jukes-Browne, B.A., P.G.S8. 325 
only shows a few svattered particles. For this drawing I am in- 
debted to Mr. W. Hill, F.G.S., and the wood engraving is a 
testimony of Mr. J. D. Cooper’s skill in that art. 
The higher part of the Lower Chalk consists of alternating soft 
and hard beds, some of the latter being very hard and weathering 
out as steps in the old cart tracks which lead up to the downs above 
Heddington, Roundway, Allington, Eastcott, and other places. 
Under the microscope these hard beds are seen to consist largely of 
fragments of Jnoceramus shell, a few foraminifera, and a few spicules. 
There is no globular silica, but a large proportion of the shell 
fragments have been silicified, that is to say, the fibrous arragonite 
of the original shell has been replaced by minutely crystalline silica 
(or chalcedony). 
6. Middle Chalk. On the top of Roundway Hill, near the 
plantation, there are some old pits in a very hard white rubbly Chalk. 
This is the bed which is now called the Melbourn Rock, and forms 
the base of the Middle Chalk. It occurs at the same horizon and 
is of the same hard rough nodular character all over England, from 
Yorkshire to Dorset and from Dorset to Dover, and it has also been 
found in the deep wells under London. I have not the slightest 
doubt that it runs all round the Vale of Pewsey, and underlies the 
whole of Salisbury Plain, and one of my colleagues is now engaged 
in tracing it over the country between Marlborough and Calne. 
The Melbourn Rock is not a compact homogeneous Chalk, but 
consists of layers of small nodules of a hard compact chalk em- 
bedded in a chalk of coarser texture and largely composed of frag- 
ments of Inoceramus shell. This structure is shown very clearly in 
the slide from which Fig. 4 is taken, and is fairly well shown in 
the engraving, which has been made from a photograph taken by 
Mr. W. Freshwater. The dark portions are the nodules of compact 
Chalk, and the other part is the shelly Chalk, one large fragment 
of shell extending right across the lower part of the figure. 
The higher part of the Middle Chalk is a typical white Chalk, so 
pure, and containing so few flints or fragments of shell that it is 
often used to make whitening. The material of which it consists is so 
fine that even under the microscope it looks like fine white powder, 
