MIDDLE CHALK, MICROSCOPIC STRUCTURE. 
499 
CHAPTER, XLIIX. 
THE MICROSCOPIC STRUCTURE OF THE 
MIDDLE CHALK. 
By W. Hill. 
The Middle Chalk, while showing some differences of character 
when considered in vertical section, shows little change laterally 
over the whole area included in this memoir. It will, therefore, 
be unnecessary to describe its aspect in different districts, except 
to notice some local peculiarity. Its basement bed is a hard white 
or yellowish-white rough nodular chalk, known as the Melbourn 
Rock. Above this bed the chalk becomes less nodular and more 
regularly bedded, though in some localities layers of nodules 
continue to occur for 40 or 50 feet, or even more. Many shell- 
fragments can be seen in it, and its weathered surfaces have a some¬ 
what gritty touch. As one passes to the Terebratulina zone the 
chalk gradually assumes the character of a soft white pulverulent 
limestone, a condition which generally continues to the Chalk 
Rock, though sometimes in the South of England it becomes 
harder and more nodular again towards the upper part of the 
zone. 
The Zone of Rhynchonella Cuvieri. 
A. Melbourn Rock and Basement Beds. f 
Macroscopic Aspect. 
The appearance of the Melbourn Rock is usually that of a hard 
white or yellowish-white rough nodular chalk, markedly seamed 
and veined by greenish-grey. Smoothed hand specimens show 
that the veins often define more or less clearly nodules or lumps 
of chalk of varying size, which are not infrequently coated thinly 
with a reddish-brown deposit. The nodules are in some cases 
thickly packed together in beds or layers, in others they are widely 
separated and distinct from each other, and sometimes spaces of 
comparatively smooth chalk separate well-marked nodular beds, but 
though the rock may appear to be smooth and without nodules, 
it invariably splits on weathering with a rough lumpy nodular 
fracture. There is great variation in the sharpness of the outline 
of the nodules. Sometimes they are seen as angular or sub- 
angular fragments of chalk, very clearly defined from the sur¬ 
rounding shelly matrix, and from this there is every gradation 
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