By A. J. Jules-Browne, B.A., F.G.S. 



325 



only shows a few scattered 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 Inoceraw.us shell, a few t 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 of the original shell has been replaced by minutely crystalline silica 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 Round way 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, 



