364 
THE CRETACEOUS ROCKS OF BRITAIN, 
they vary greatly, from a minute particle not larger than a pin’s 
head ..... to the rounded lump to which young oysters may 
often be found adhering.” He also points out that the shapes and 
outlines of the nodules can be seen very well if a hard specimen 
of the rock be ground down and smoothed to a level surface. 
Fossils are seldom found in the lower part of the rock except 
the small attached valves of young oysters which occasionally 
occur on the nodules ; but Rhynchonella Cuvieri and a few other 
species occur in the upper beds, ranging thence through the zone 
of Rhynchonella Cuvieri. 
The hard and nodular character of the Melbourn Eock enables 
it to resist atmospheric disintegration better than the bloeky and 
marly Lower Chalk, so that its outcrop generally makes a well 
defined feature. When the Middle Chalk is fairly thick and 
the dip is slight there is generally a double escarpment, the first 
slope being capped by the Melbourn Eock and the second slope 
by the Chalk Eock. The boundary line of the Melbourn Eock 
has been engraved on all the maps of Chalk areas published by 
the Geological Survev since 1880. 
The Melbourn Eock is thickest in Kent and Sussex, and was 
described as the “ Grit Bed ” by Mr. Price. It is traceable west¬ 
ward into Dorset and northwards into Norfolk, but ceases to be 
distinguishable from the rest of the zone in the north-west of the 
latter countv. 
c/ 
Upwards the Melbourn Eock passes into a rather hard and rough 
kind of chalk, somewhat yellowish, and lying in regular beds which 
are frequently divided by layers of loose marly chalk. Nodules 
like those in the Melbourn Eock are still abundant in the lower 
part of this chalk, but gradually disappear in the higher beds. 
The shells of Inoceramus mytiloides are abundant in these beds, 
and their broken fragments, large and small, make up a consider¬ 
able proportion of its mass. Scattered nodules of flint sometimes 
occur in the higher beds, but they are never frequent. 
Rhynchonella Cuvieri is common throughout these beds, and 
in many localities Car diaster pygmceus , Discoidea Dixoni , and 
Galerites sub rotundas may be found. Micrasters are rarely found 
in this zone, except in Devonshire, where Micraster corbovis and 
M. Leskei are not uncommon in association with large Cardiaster 
pygmceus quite at the base of the zone. The characteristic Am¬ 
monite of the.zone is Am. [ Acanthoceras\ nodosoides (see Fig. 56), 
though it canuot be called common. 
The thickness of this zone, including the Melbourn Eock, does 
not vary greatly in the southern and midland counties, where it is 
generally between 60 and 80 feet thick, the chalk rapidly losing 
its nodular and shelly character at this distance from the base. 
When traced northward, however, it diminishes in thickness, being 
onlv 15 feet thick in Lincolnshire- and still less in Yorkshire. 
On the Devon coast also it is in some places only 14 feet in 
thickness. 
